This is the 791st most frequent Turkish word.
birisi
"Birisi" means "someone" or "somebody" in English.
In this sentence, 'birisi' is used as an indefinite pronoun referring to an unspecified person.
Birisi kapıyı çaldı.
Someone knocked on the door.
In this case, 'birisi' denotes an unidentified individual involved in the activity.
Bahçede birisi piyano çalıyor.
Someone is playing the piano in the garden.
Here, 'birisi' implies an indefinite person responsible for the task at hand.