This is the 3087th most frequent Turkish word.
bilir
"Knows" or "he/she/it knows."
Here, 'bilir' functions as a modulated verb indicating knowledge or expertise in a subject.
O, bu konuyu çok iyi bilir.
He/She knows this subject very well.
In this case, 'bilir' is part of the compound noun 'bilir kişi' meaning 'expert' literally translated as 'knowing person'.
Bilir kişi bize tavsiyelerde bulundu.
The expert gave us advice.
Here, 'bilir' is used in the rhetorical question phrase 'kim bilir,' which signifies uncertainty or speculation equivalent to 'who knows.'