Monads are like burritos. This is explained very well at http://blog.plover.com/prog/burritos.html.

In short they are way to automate building new types out of old types in such a way that code written for the old types can immediately be turned into code written for the new types. This allows a lot of otherwise repetitive code that you might otherwise wind up writing to be pushed to your type system. Which in turn makes it feasible to build a richer type system.

Haskell takes this idea to an extreme.