double exclamation marks in haskell -

i have code:

ghci>let listoffuns = map (*) [0..] ghci>(listoffuns !! 4) 5 20 

1. what !! mean?

   i saw example double exclamation this:

   ghci> [1,2,3,4]!!1 ghci> 2

but seems don't apply question example.

1. how understand function. need explanations.

!! indexes lists. takes list , index, , returns item @ index. if index out of bounds, returns ⊥.