What does the phrase A monad is a monoid in the class of endofunctors mean?
The phrase A monad is a monoid in the class of endofunctors is a statement from category theory that describes the relationship between monads, monoids, and endofunctors.

In category theory, a monoid is a mathematical structure consisting of a set equipped with an associative binary operation and an identity element. An endofunctor is a functor that maps a category to itself, meaning it takes objects from the category and maps them to other objects in the same category.

A monad is a special type of endofunctor that has two additional operations, called unit and join, that satisfy certain axioms. The unit operation takes an object from the category and maps it to an object in the monad, while the join operation takes two nested objects in the monad and flattens them into a single object.

The phrase A monad is a monoid in the class of endofunctors means that a monad can be thought of as a monoid in the category of endofunctors. This statement is saying that the operations of a monad (unit and join) have the same algebraic structure as the operations of a monoid, but in the context of endofunctors. This insight can help in understanding the properties and behavior of monads, as well as their relationships with other mathematical structures.