La reunión se retrasó por la lluvia.

In La reunión se retrasó por la lluvia, se does not literally mean itself.

Here retrasarse is being used as a pronominal / reflexive-like verb meaning to be delayed / to run late.

• retrasar (non‑reflexive): to delay something
  
  • La lluvia retrasó la reunión. – The rain delayed the meeting.
• retrasarse (pronominal): to be delayed / to run late
  
  • La reunión se retrasó. – The meeting was delayed / ran late.

So se here is part of a very common pattern in Spanish where the reflexive form gives a more intransitive, “it got delayed” meaning, similar to an English passive:
La reunión se retrasó ≈ The meeting was delayed / ended up being late.

Se retrasó uses the preterite because it refers to a completed event at a specific time: the meeting (which took place) ended up being delayed.

• La reunión se retrasó por la lluvia.
  → It was delayed (finished, one completed event, like “it got delayed”).

If you used the imperfect (se retrasaba), it would sound like you’re talking about an ongoing or habitual situation in the past, for example:

• La reunión se retrasaba todos los lunes.
  → The meeting used to run late every Monday.

So for one specific meeting that was delayed, the preterite se retrasó is the natural choice.

It’s not wrong, but it’s less common in everyday speech.

• La reunión fue retrasada por la lluvia.
  is a true passive voice (with ser + past participle), similar to English:
  → The meeting was delayed by the rain.

In Spanish, for this type of sentence, people much more often use:

• La reunión se retrasó por la lluvia. (pronominal / “se-passive”)
• La lluvia retrasó la reunión. (active voice: the rain delayed the meeting)

In Latin American Spanish, La reunión se retrasó por la lluvia sounds more natural and conversational than fue retrasada.

Both are correct and mean essentially the same thing.

1. La reunión se retrasó por la lluvia.

   • Focus is on the meeting as the thing that ended up delayed.
   • Uses the pronominal form retrasarse, a very common way to say was delayed / ran late.

2. La lluvia retrasó la reunión.

   • Focus is on the rain as an active cause.
   • Uses the normal verb retrasar with la lluvia as the subject.

Nuance:

• Se retrasó: “the meeting got delayed” (very neutral and common).
• La lluvia retrasó la reunión: slightly more emphasis on the rain doing the delaying.

Because in Spanish, cause is normally expressed with por, a causa de, or debido a, not porque de.

• Correct:

  • La reunión se retrasó por la lluvia.
  • La reunión se retrasó a causa de la lluvia.
  • La reunión se retrasó debido a la lluvia.

• Incorrect / unnatural:

  • ✗ La reunión se retrasó porque de la lluvia.

Porque introduces a clause:

• La reunión se retrasó porque llovía mucho.
  → ...because it was raining a lot.

But when you put a noun directly after, you use:

• por la lluvia
• a causa de la lluvia
• debido a la lluvia

Para la lluvia would normally mean something like for the rain (a purpose), which is not the idea here.

Yes, that’s correct Spanish.

• La reunión se retrasó por la lluvia.
• Por la lluvia, la reunión se retrasó.

Both are grammatical. The first is more neutral, everyday word order.
Putting Por la lluvia at the beginning just adds a bit of emphasis to the cause:

Because of the rain, the meeting was delayed.

Yes, both are possible:

• La reunión se retrasó por la lluvia.
• La reunión se retrasó por lluvia.

The version with la (por la lluvia) is more specific: because of the rain (the rain that happened).

Without the article (por lluvia) feels more generic or categorical: “due to rain.”
You often see this shorter form in announcements, signs, or headlines:

• Vuelo cancelado por lluvia. – Flight canceled due to rain.
• Partido suspendido por lluvia. – Match suspended due to rain.

In everyday speech, por la lluvia is more common, but por lluvia is not wrong.

Yes, reunión is feminine:

• la reunión – the meeting
• una reunión – a meeting
• esta reunión – this meeting

Plural:

• las reuniones – the meetings
• unas reuniones – some meetings

Examples:

• Las reuniones se retrasaron por la lluvia.
  → The meetings were delayed because of the rain.

In some Latin American countries, especially Mexico, junta is commonly used in everyday speech to mean meeting (often a work meeting):

• Tenemos junta a las 9. – We have a meeting at 9.
• La junta se retrasó por la lluvia.

However:

• reunión is standard and understood everywhere in the Spanish-speaking world.
• junta can sound more informal or regional, depending on the country.

If you want to be safe and neutral for all of Latin America, reunión is the best choice.

Yes, retrasarse is commonly used for people and things that are late:

• Me retrasé por la lluvia. – I was delayed / I was late because of the rain.
• Nos retrasamos por el tráfico. – We were delayed by the traffic.
• El tren se retrasó una hora. – The train was delayed by an hour.
• La película se retrasó quince minutos. – The movie started 15 minutes late.

So retrasarse is a very useful verb for talking about lateness or delays.

They all have to do with delays, but are used slightly differently.

1. retrasar / retrasarse

   • to delay / to be delayed, run late
   • Very common in many countries.
   • La reunión se retrasó por la lluvia.
   • El tren se retrasó dos horas.

2. atrasar / atrasarse

   • Similar to retrasar in many contexts; also used for clocks.
   • El partido se atrasó por la lluvia.
   • Atrasamos el reloj una hora.

3. demorar / demorarse

   • Also used for delays, sometimes a bit more formal, but very common in parts of Latin America.
   • El vuelo se demoró por la lluvia.
   • Me demoré en llegar.

4. aplazar / posponer

   • These usually mean to postpone (move to a later date/time), not just start late.
   • Aplazaron la reunión para mañana. – They postponed the meeting to tomorrow.
   • Pospusimos la reunión una semana.

In your sentence, se retrasó suggests more that the meeting started late / was delayed, not necessarily moved to another day. If it were postponed to another day, you’d likely say:

• La reunión se pospuso por la lluvia.
• La reunión se aplazó por la lluvia.

reunión

• Syllables: re-u-nión (3 syllables; re – u – nión)
• The accent mark on ó means the stress is on the last syllable: re-u-NIÓN.
• The e-u is pronounced as two separate vowels, not one blended sound.

lluvia

• Syllables: llu-via (2 syllables).
• In most of Latin America, ll is pronounced like y in yes.
• So it sounds roughly like YOO-vya (depending on accent).

Yes, but the meaning changes slightly:

• La reunión se retrasó por la lluvia.
  → The meeting was delayed / started late due to the rain.
• La reunión se pospuso por la lluvia.
  → The meeting was postponed / moved to a later time or date because of the rain.

Use:

• se retrasó if it still happened that day, just later than planned.
• se pospuso if it was rescheduled to another time or day.