Biri kapıyı çaldı.
English: Someone knocked on the door.
In Turkish, 'belgisiz zamirler' are indefinite pronouns. They refer to people or things in a general way, without specifying exactly who or what.
Use belgisiz zamirler when you want to talk about people or things in a general or indefinite way. For example, when you don't know exactly who or what you are referring to, or when it is not important to be specific.
Biri kapıyı çaldı.
English: Someone knocked on the door.
Hiçbiri gelmedi.
English: None of them came.
Bazıları elma seviyor.
English: Some (of them) like apples.
Kimse evde yok.
English: No one is at home.
Herkes mutlu.
English: Everyone is happy.