What is this post talking about?

I don't know what you computer science folk are talking about, but a monad in philosophy in one sentence is: a simple object (meaning indivisible) that causally interacts with other objects by observing its environment.

It is contrasted to other (more standard) views of objects that act upon other objects to make them move. Monads can't operate on other objects, they can only operate on themselves.

For more information google "Leibniz"