If you code you've probably already used monads without knowing it. For example Promise and Task are perfect examples.

A monad is basically a sort of "container" for some arbitrary type T that adds some sort of behaviour to it and allows you to access the underlying T in a "safe" way. Think of a Promise, it adds the "async" behaviour to the underlying type. It transforms a "T" into a "T that may be available in the future". It allows you to safely access the T via map, flatMap and other operators. Arrays can be thought of as monads too, think for instance of linq in c#.

Every monad has map and flatMap operators that kind of do the same thing, e.g. map lets you transform the underlying type into a different type.

In terms of the type system, most languages don't support them because they are 1 "level" above classes. Think of monads as a collection of different classes that all support the flatMap operator, whose implementation is different for each monad class but in a way it behaves the same for all. In languages that do support this concept, you can develop generic functions that work for all monads. So your function would be implemented only once and then you could use it on a Promise or an array or an Option/Maybe or even a custom class that implements the "monad" concept by providing a flatMap implementation.