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Quick summary:

• Every time a message is published on the keys topic, the keys_cb callback is triggered, and the argument msg contains that latest message
• Based on the data in the latest keys message, the keys_cb updates the value of the global variable g_last_twist.
• The while loop you referenced runs at a rate of 10Hz and continually publishes the value of g_last_twist
• The callback keys_cb is also passed a reference to the twist_pub publisher via the callback_args keyword argument in the constructor for a rospy.Subscriber. The callback also publishes the latest value of g_last_twist using its reference to the twist_pub-- presumably so you don't need to wait until the next iteration of the main while loop to get the value published.