This is the 12802nd most frequent Turkish word.
izlediğini
"İzlediğini" means "that (you/he/she/they) watched or are watching."
In this context, 'izlediğini' means 'what you were watching', indicating an action performed by the addressed individual that was not known by the speaker.
Senin izlediğini bilemezdim.
I couldn't have known what you were watching.
Here, 'izlediğini' refers to 'what you watched', used as the object to express that the action is known to everyone.
İzlediğini herkes biliyor.
Everyone knows what you watched.
Here, 'izlediğini' translates to 'what he/she watched', used to express an action that has been observed by the subject, combined with a following verb to describe the subject's opinion on it.