[Haskell-beginners] how to undertand the function join (*)

Hello Marcus, what does join (*) do? (*) :: Num a => a -> a -> a join (*) :: Num a => a -> a when we feed a number to join (*),for instance; λ> : join (*) 3 9 it seems thata join (*) become a square function. what does join do to (*) to make that happen?