A reunião foi rápida.

A is the feminine singular definite article, meaning the.

So A reunião means the meeting, usually a specific meeting the speaker has in mind.

In Portuguese, articles are used very often. If you drop the article and say just Reunião foi rápida, it sounds incomplete in normal speech, unless it is a headline or note style.

Yes, reunião is feminine.

You can tell in this sentence because:

• the article is a
• the adjective is rápida, also feminine

A useful habit is to learn nouns together with their article:

• a reunião
• not just reunião

Also, the ending -ão does not reliably tell you the gender. Some -ão nouns are masculine, and some are feminine.

Here, foi is the third-person singular preterite form of ser.

So grammatically it means:

• ser = to be
• foi = was

It matches the subject a reunião, which is singular.

One thing that confuses learners: foi can also be the past form of ir (went). But in this sentence, it clearly means was, because it is followed by the adjective rápida.

Foi is used because the meeting is being described as a completed event.

A reunião foi rápida gives a simple finished past idea: the meeting happened, and overall it was quick.

Era is the imperfect form of ser, and it is usually used for:

• background description
• repeated/habitual situations
• ongoing past context

So for one specific finished meeting, foi rápida is the normal choice.

With events like meetings, classes, trips, parties, and movies, Portuguese very often uses ser to describe their general character or duration:

• A reunião foi rápida
• A aula foi longa
• A festa foi ótima

This is the natural pattern.

Using estar here with rápida would sound odd. In this sentence, Portuguese treats quick as a characteristic of the event, so ser is preferred.

Because adjectives in Portuguese usually agree with the noun they describe in gender and number.

Here:

• reunião is feminine singular
• so the adjective must also be feminine singular: rápida

Compare:

• A reunião foi rápida
• O encontro foi rápido

Because the sentence describes the meeting itself, not the way an action was done.

This structure is like:

• The meeting was quick

So Portuguese uses an adjective after ser:

• foi rápida

If you wanted an adverb, you would normally need a different verb, for example:

• A reunião terminou rapidamente = the meeting ended quickly

That is a different sentence structure.

They help show pronunciation.

• In reunião, the ã shows a nasal vowel sound.
• In rápida, the acute accent on á shows that the stress falls on the first syllable: RÁ-pi-da.

So the marks are not optional spelling decorations; they are part of the correct written form.

A rough English-friendly guide is:

a heh-oo-nee-OWN foy HAH-pee-da

A few helpful notes:

• foi sounds roughly like foy
• the r at the beginning of reunião and rápida is often pronounced like an English h in many Brazilian accents
• -ão is nasal, which English does not really have

So the overall rhythm is roughly:

a re-u-ni-ÃO foi RÁ-pi-da

The normal, neutral order is:

A reunião foi rápida.

That is the best version to learn first.

Other orders are possible, but they are marked or stylistically special:

• Foi rápida a reunião. = possible, but more emphatic or literary

Also, be careful:

• a reunião rápida is not the same thing as the full sentence
• it means the quick meeting, just a noun phrase

Yes, but the meaning changes.

• A reunião foi rápida = the meeting was quick
  a specific meeting

• Uma reunião foi rápida = a meeting was quick
  an unspecified meeting

In most basic example sentences, a reunião sounds more natural because it refers to a particular meeting already understood from context.