Collaborative filtering is where you have information about who bought what, or who liked what﹣ it’s basically something where you have something like a user, a reviewer, or whatever and information about what they’ve bought, what they’ve written about, or what they reviewed. L4

the cold start problem is that the time you particularly want to be good at recommending movies is when you have a new user, and the time you particularly care about recommending a movie is when it’s a new movie. But at that point, you don’t have any data in your collaborative filtering system and it’s really hard.