A "monad" is a description of something so general it covers lists, things that might not exist, things that might fail, and things that might happen in the fut

A "monad" is a description of something so general it covers lists, things that might not exist, things that might fail, and things that might happen in the future. It is a concept from category theory, which is the branch of mathematics that deals with saying as little as possible about as much as possible. You don't have to worry about any of this unless a teacher tells you to. (And if they do, you can just leave.)