Widownia zaczęła klaskać po refrenie.

Here, widownia means the audience.

A useful nuance: widownia can also mean the auditorium / the seating area in some contexts, but in this sentence it clearly means the people watching.

So Widownia zaczęła klaskać = The audience started clapping.

Because widownia is a singular feminine noun in Polish.

Even though the audience refers to many people in real life, grammatically it is treated as one collective thing, so the verb agrees with the noun:

• widownia — feminine singular
• zaczęła — past tense, feminine singular

So Polish matches the grammar of the noun, not the number of people inside the group.

Zaczęła is the past tense of zacząć (to begin / to start), in the:

• 3rd person
• singular
• feminine

That is why it matches widownia.

Related forms:

• zaczął — he/it started
• zaczęła — she/it started
• zaczęło — it started (neuter)
• zaczęli / zaczęły — they started

Because after verbs like zacząć (to start/begin), Polish commonly uses the infinitive of the second verb.

So:

• zacząć klaskać = to start clapping
• literally: started to clap

This works like:

• zaczął mówić — he started speaking
• zaczęła śpiewać — she started singing
• zaczęli biec — they started running

Klaskać means to clap or to applaud with your hands.

In this sentence, it means to start applauding / clapping.

A useful note:

• klaskać is imperfective, so it describes the action of clapping as an activity.
• If you want a more completion-focused perfective partner, Polish often uses zaklaskać in some contexts, but zacząć klaskać is the natural choice for start clapping.

Because po here means after, and with that meaning it takes the locative case.

So:

• base form: refren = chorus / refrain
• locative after po: po refrenie = after the chorus

This is a very common pattern:

• po obiedzie — after lunch
• po pracy — after work
• po filmie — after the film
• po refrenie — after the chorus

Yes, in this kind of sentence refren means chorus or refrain in a song.

So po refrenie means:

• after the chorus
• after the refrain

In many learning contexts, chorus is the most natural English translation.

No. Po refrenie means after the chorus, not during it.

If you wanted during the chorus, Polish would use a different expression, for example:

• w czasie refrenu — during the chorus
• podczas refrenu — during the chorus

So po is specifically marking something that happens later.

Yes, Polish word order is fairly flexible, although the original sentence is the most neutral.

Neutral order:

• Widownia zaczęła klaskać po refrenie.

You could also say:

• Po refrenie widownia zaczęła klaskać.

That version puts more focus on after the chorus.

Both are correct, but the first one is the plain, standard way to say it.

Because zacząć is perfective, and Polish usually uses the perfective verb when talking about a completed change of state like starting.

So:

• widownia zaczęła klaskać = the audience started clapping

This presents the beginning as a single completed event.

By contrast, zaczynać is imperfective and is used in contexts like repeated, habitual, or ongoing background situations:

• Widownia zawsze zaczynała klaskać po refrenie.
  The audience always started clapping after the chorus.

So in a simple one-time past event, zaczęła is the natural choice.

No. Another very common word is publiczność.

Both can mean audience, but they are not always identical in feel:

• widownia can suggest the audience in a theater, concert hall, cinema, etc.
• publiczność is also very common for an audience and often feels a bit broader or more general

In this sentence, widownia sounds completely natural.

For example:

• Publiczność zaczęła klaskać po refrenie.
• Widownia zaczęła klaskać po refrenie.

Both are possible.

A rough English-friendly pronunciation is:

vee-DOV-nyah zah-CHEN-wah KLAS-katch po reh-FREH-nyeh

A few helpful points:

• w sounds like English v
• cz sounds like ch in chop
• ć at the end of klaskać is a soft ch-like sound
• rz in zaczęła sounds like zh (like the s in measure)
• ń in widownia is a soft ny sound

This is only approximate, but it should help you get close.