For monads, learn the following concepts in order: Category Functor Natural transformation Adjoint functor That's it. If you don't understand what monads

For monads, learn the following concepts in order: Category Functor Natural transformation Adjoint functor That's it. If you don't understand what monads are you should probably wait with topos theory. It quite abstract and you should probably be comfortable with both algebraic geometry and category theory before you approach it