Hello Nika,
"It's all Greek to me" = It is all incomprehensible to me
The idiomatic phrase comes from Shakespeare's play " Julius Caesar". The idiom "It's all Greek to me" means it is mysterious to me as Greek writing. I can't grasp it nor understand it and it doesn't make any sense at all. It goes for both things said and written.
" I've tried reading the manual but it's all Greek to me."
" He is speaking a language I can't understand, It's all Greek to me."