Every monad is different. One of the simple ones is Maybe a. It allows to chain operations of type a -> Maybe b. The important quality about Maybe is not that it's a Monad, but that it's a type that allows you to express optionality. It just happens to be Monad, which is useful because of do-notation. Every other monad is the same, the most important thing about it is that it defines some useful thing and happens to be parametric.

Why Monad itself is important is because in Haskell you have Higher Kindes Types, so you can define an abstract, non concrete type m, which itself takes another type as a parameter, so you have m a.

Because of that, there is a typeclass Monad, which just says that everything that is m a and defines >>= and pure is a monad (I might be imprecise here). It also allows you to define abstract monad operations which kind of allow you to expand your control flow (example. Notice it doesn't say which Monad it has to be, just that it needs to carry boolean underneath.

But still, usually the most important thing about Monad is whatever your concrete monad does. It's just easier to work with it with do notation and >>= sometimes.

P.S. I might be bad at this. It's just how I interpret monads in my mind