When a sentence is started with "If I were you, I'd...", the speaker is recommending a certain action or inaction to the listener. For example:
Jack has an exam tomorrow, but he missed a lot of classes. His roommate, Peter, says: "If I were you, I'd study very hard tonight!", meaning that Jack should study very hard tonight.
Literally it means, "If I was in your place, I would...". The speaker (the person who says "If I were you...") tells the listener what he (the speaker) would do, if he was in the same situation as the listener.