Why does Subscriber have twist_pub ?

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I am referring this book called "Programming with ROS" by Morgan Quigley. I am having difficulty understanding this particular code :

 Example 8-3. keys_to_twist_using_rate.py
#!/usr/bin/env python
import rospy
from std_msgs.msg import String
from geometry_msgs.msg import Twist
key_mapping = { 'w': [ 0, 1], 'x': [0, -1],
                'a': [-1, 0], 'd': [1, 0],
                's': [ 0, 0] }
g_last_twist = None
def keys_cb(msg, twist_pub):
    global g_last_twist
    if len(msg.data) == 0 or not key_mapping.has_key(msg.data[0]):
       return # unknown key
    vels = key_mapping[msg.data[0]]
    g_last_twist.angular.z = vels[0]
    g_last_twist.linear.x = vels[1]
    twist_pub.publish(g_last_twist)
if __name__ == '__main__':
   rospy.init_node('keys_to_twist')
   twist_pub = rospy.Publisher('cmd_vel', Twist, queue_size=1)
   rospy.Subscriber('keys', String, keys_cb, twist_pub)
   rate = rospy.Rate(10)
   g_last_twist = Twist() # initializes to zero
   while not rospy.is_shutdown():
          twist_pub.publish(g_last_twist)
          rate.sleep()

So my question is : 1. Why does rospy.Subscriber('keys', String, keys_cb, twist_pub) have twist_ pub? 2. What is exactly happening in the while loop ?

while not rospy.is_shutdown():
              twist_pub.publish(g_last_twist)
              rate.sleep()