This is the 4612th most frequent Turkish word.
bildiğini
"That which you know."
In this sentence, 'bildiğini' is used as a noun phrase meaning 'what he/she/they know'.
Onun bildiğini sanmıyorum.
I don't think he/she/they know(s) it.
Here, 'bildiklerini' is the object, referring to 'the things you know' in an imperative sentence.
Bildiklerini paylaşmaktan çekinme.
Do not hesitate to share what you know.
'Bildiğini' in this sentence functions as an indirect object, meaning 'what you believe to be true'.