What are some AI/ML concepts or problems you found difficult while learning?

I am also collecting such questions for a small academic project related to an AI question answering system. If anyone is interested in contributing, you can submit here. It takes less than 2 minutes and does not collect personal information: https://docs.google.com/forms/d/e/1FAIpQLSdQeFTqqDncjZMk0L4vAs-1S1WGMPMQUmR7iHBO-RjazSwxFg/viewform?usp=dialog