Something is all Greek to you when it is so hard to understand that it is like reading a foreign (Greek) language. Example: "Can you read these instructions?" Reply: "No. It's all Greek to me."

When you hear or read something that you don't understand at all, you can say that it is all Greek to you. Example: "Did you understand what he just said?" Reply: "No, it was all Greek to me." Another example: "I don't understand this contract. It's all Greek to me."