O que if __name__ == “__main__” faz?

O que acontece durante uma chamada  if __name__ == "__main__":?

# Exemplo de Threading
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 = thread.allocate_lock()
thread.start_new_thread(myFunction, ("Thread #: 1", 2, lock))
thread.start_new_thread(myFunction,