Birisi kapıyı çaldı.
English: Someone knocked on the door.
In Turkish, 'belgisiz zamirler' are indefinite pronouns. They are used to refer to people or things in a general, non-specific way.
Use belgisiz zamirler in Turkish when you talk about people or things without naming them exactly. They are helpful when you don't know, don't want to say, or don't need to specify who or what you mean.
Birisi kapıyı çaldı.
English: Someone knocked on the door.
Kimse gelmedi.
English: No one came.
Bazıları bu filmi seviyor.
English: Some (people) like this movie.
Herkes hazır mı?
English: Is everyone ready?
Hiçbiri doğru değil.
English: None of them is correct.