Was macht if __name__ == "__main__"?

Was passiert während eines Aufrufs von if __name__ == "__main__":?

# Threading Example
import time, thread
def myfunction(string, sleeptime, lock, *args):
    while 1:
        lock.acquire()
        time.sleep(sleeptime)
        lock.release()
        time.sleep(sleeptime)
if __name__ == "__main__":
    lock =