What does '@' mean in Haskell?

It is used in pattern matching. Now node variable will refer to the entire Node data type for the argument Node a b c l r. So instead of passing to the function as Node a b c l r, you can use node instead to pass it up.

A much simpler example to demonstrate it:

data SomeType = Leaf Int Int Int | Nil deriving Show

someFunction :: SomeType -> SomeType
someFunction leaf@(Leaf _ _ _) = leaf
someFunction Nil = Leaf 0 0 0

The someFunction can also be written as:

someFunction :: SomeType -> SomeType
someFunction (Leaf x y z) = Leaf x y z
someFunction Nil = Leaf 0 0 0

See how simpler was the first version ?

Using @t as a type indicator

Besides the argument pattern matching usage described in the answer of @Sibi, in Haskell the "at" character ('@', also known as an arobase character) can be used in some contexts to force a typing decision. This is mentioned in the comments by @Josh.F.

This is not part of the default language features, and is known as the Type Application Haskell language extension. In summary, the extension allows you to give explicit type arguments to a polymorphic function such as read. In a classic .hs source file, the relevant pragma must be included:

{-#  LANGUAGE TypeApplications  #-}

Example:

$ ghci
GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
 λ> 
 λ> let x = (read @Integer "33")

 <interactive>:4:10: error:
    Pattern syntax in expression context: read@Integer
    Did you mean to enable TypeApplications?
 λ> 
 λ> :set -XTypeApplications
 λ>
 λ> let x = (read @Integer "33")
 λ>
 λ> :type  x
 x :: Integer
 λ> 
 λ> x
 33
 λ> 

Further details

For the read polymorphic function, the type indicator introduced by @ relates to the type of the result returned by read. But this is not generally true.

Generally speaking, you have to consider the type variables that appear in the type signature of the function at hand. For example, let's have a look at the fmap library function.

fmap :: Functor ft => (a -> b) -> ft a -> ft b

So here, we have 3 type variables, in order of appearance: ft, a, b. If we specialize fmap like this:

myFmap = fmap  @type1  @type2  @type3

then type1 will relate to ft, type2 will relate to a and type3 will relate to b. Also, there is a special dummy type indicator @_ which means: “here, any type goes”.

For example, we can force the output type of fmap to be Integer and the functor to be the plain list [], leaving the input type a unspecified:

 λ> 
 λ> myFmap = fmap  @[]  @_  @Integer
 λ> 
 λ> :type myFmap
 myFmap :: (_ -> Integer) -> [_] -> [Integer]
 λ> 

As for the read function, its type is:

read :: Read a => String -> a

So there is only room for one type indicator, and it relates to the type of the result returned by read, as displayed above.