Groovy , what does -> mean

It is used to separate where you declare bindings for your closure from the actual code, eg:

def myClosure = { x, y -> x + y }

the part before -> declares that the closure has two arguments named x and y while the second part is the code of the closure.

You can omit it in closures with just one parameter, in that case the it variable is assumed:

[1, 2, 3, 4].each{ println it*2 }

but you could also do

[1, 2, 3, 4].each{ lol -> println lol*2 }