($!) is strict function application. That is, it evaluates the argument before evaluating the function.
This is contrary to normal lazy function application in Haskell, e.g. f x or f $ x, which first start to evaluate the function f, and only compute the argument x if it is needed.
For example succ (1 + 2) will delay the addition 1 + 2 by creating a thunk, and start to evaluate succ first. Only if the argument to succ is needed, will 1 + 2 be evaluated.
However, if you know for sure that the argument to a function will always be needed, you can use ($!), which will first evaluate the argument to weak head normal form, and then enter the function. This way, you don’t create a whole big pile of thunks and this can be more efficient. In this example, succ $! 1 + 2 would first compute 3 and then enter the function succ.
Note that it is not always safe to just replace normal function application with strict function application. For example:
ghci> const 1 (error "noo!")
1
ghci> const 1 $! (error "noo!")
*** Exception: noo!