What do you call this type of parameter?

[league55]

In some code I've seen stuff like this:

def some_function(a -> int):
    #do something

I thought a -> int was called a decorator, but a decorator seems to be this:

def some_function(func):
    def some_func(*args):
        #do something to func
        return func(*args)
    return some_func

@some_function
def some_other_function(params):
    #do something

So if a -> int is not a decorator, what's it called and what does it do?