Flask : What exactly is @app

It is a decorator. When decorated by @app.route('/') (which is a function), calling index() becomes the same as calling app.route('/')(index)().

Here is another link that can explain it, in the python wiki.

The @ is telling Python to decorate the function index() with the decorator defined in app.route().

Basically, a decorator is a function that modifies the behaviour of another function. As a toy example, consider this.

def square(func):
    def inner(x):
        return func(x) ** 2
    return inner

@square
def dbl(x):
    return x * 2 

Now - calling dbl(10) will return not 20, as you'd expect but 400 (20**2) instead.

This is a nice step-by-step. explanation of decorators.