Explanation.hs

f :: String -> [String]
f x = [x ++ "!", x ++ "."]

["Hi"] >>= f >>= f
-- is:
(["Hi"] >>= f) >>= f

-- Which, according to the Prelude implementation of the List monad...:

instance  Monad []  where
    m >>= k          = concat (map k m)
    return x         = [x]
    fail s           = []

-- ...does:

{- 1 -} concat (map f (["Hi"] >>= f))
{- 2 -} concat (map f (concat (map f ["Hi"])))
{- 3 -} concat (map f (concat [["Hi!", "Hi."]]))
{- 4 -} concat (map f ["Hi!", "Hi."])
{- 5 -} concat [["Hi!!", "Hi!."], ["Hi.!", "Hi.."]]
{- 6 -} ["Hi!!", "Hi!.", "Hi.!", "Hi.."]

gistfile3.hs

f :: String -> [String]
f x = [x ++ "!", x ++ "."]

main = print $ ["Hi"] >>= f >>= f

Result

["Hi!!","Hi!.","Hi.!","Hi.."]