https://fsharpforfunandprofit.com/rop/ is by far my most recommended starting point, using a railway analogy. To grossly oversimplify, as far as I can tell, a monad is a type well-defined enough that you can hook up functions that work with it in a “parallel tracks” manner. So it’s great for chained functions, and reusing and weaving those functions, where you accumulate information other than the “obvious” thing to return as you go along, like errors or logging or IO.