Querying one's favorite search engine with the text "a monad is" yields this answer on StackOverflow, which explains, in excruciating mathematical symbology, what a monad in fact is. But, don't worry. Understanding monads doesn't require you to learn the nuances of such concepts as moirails and eigenfunctors. I would suggest that the concept of monads can be distilled into a clearer single sentence.
A monad is a control abstraction that defines a composition of effectful functions.
Let's unpack.