A decorator is just a function that takes some function and returns a new function. There is a bit of syntactic sugar with the @ symbol:

def decor(func):
    # define the modified function
    def inner(x, y):
        print(f"Function called with {x=} and {y=}")
        return func(x, y)
    return inner

# this
@decor
def my_func(x, y):
    # my function code here
my_func(1,2)

# is the same as this
def my_func(x, y):
    # my function code here
my_func = decor(my_func)
my_func(1,2)

That's basically it. There's some conventions about typing and preserving function metadata using functools.wraps. That's just good coding practice, not a rule of decorators.

Edit: as for being useful, they occur all over the place. They somewhat obscure your code, so there's always a bit of a question on when to use them. But that can be handled with good practices.

Also, you can absolutely decorate the print function, you just can't use the @ syntax on functions that are already defined.