Using global in Python

Hello, I'm learning Python and after C++ I absolutely don't understand the principle of using global variables. If I specify global for a variable inside a def, can I use its value outside this function? I definitely can't get it to work. Maybe it's just because I'm a beginner and obvious things are still too complicated for me.

def check():    global urokb_in    urokb_in = 1    with open('users.txt', 'r') as f:        line = f.readline()        f.close()    with open('progress.txt'