What is the meaning of zmq.Poller.poll( 1000 )?

I searched already but I still don't understand what's this line meaning.

Why put 1000 in the function?

full code is here

class ClientTask(threading.Thread):
    """ClientTask"""
    def __init__(self, id):
        self.id = id
        threading.Thread.__init__ (self)

    def run(self):
        context = zmq.Context()
        socket = context.socket(zmq.DEALER)
        identity = u'worker-%d' % self.id
        socket.identity = identity.encode('ascii')
        socket.connect('tcp://localhost:5570')
        print('Client %s started' % (identity))
        poll = zmq.Poller()
        poll.register(socket, zmq.POLLIN)
        reqs = 0
        while True:
            reqs = reqs + 1
            print('Req #%d sent..' % (reqs))
            socket.send_string(u'request #%d' % (reqs))
            for i in range(5):
                sockets = dict(poll.poll(1000))//HERE
                if socket in sockets:
                    msg = socket.recv()
                    tprint('Client %s received: %s' % (identity, msg))

        socket.close()
        context.term()

Here 1000 is the timeout in milliseconds to wait for an event.

See more here.