The phrase "A monad is a monoid in the class of endofunctors" is a concise way of describing a mathematical concept called a monad.

In category theory, a monad is a triple (T, η, μ), where T is an endofunctor (a function that maps a category to itself), η is a natural transformation (a mapping between functors), and μ is a natural transformation that satisfies some additional conditions.

The phrase "a monoid in the class of endofunctors" refers to the fact that the operation of combining two monads can be thought of as a monoid operation, where the elements of the monoid are the endofunctors. Specifically, the operation of combining two monads T and S is given by the composition of their endofunctors T(S(-)).
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