Quelqu’un frappe à la porte.
English: Someone is knocking at the door.
Indefinite pronouns in French are words that refer to people or things in a general or non-specific way. They do not point to a particular person or object.
Use indefinite pronouns in French when you want to talk about people or things without being specific. For example, when you don't know who or what you are talking about, or when it doesn't matter exactly who or what it is.
Quelqu’un frappe à la porte.
English: Someone is knocking at the door.
Je ne vois rien.
English: I don't see anything.
Chacun a son opinion.
English: Everyone has their own opinion.
Personne n’est venu.
English: No one came.
Plusieurs sont partis tôt.
English: Several left early.