How did townspeople become independent of the feudal system? What does the saying "town air makes you free" me?

There was a law that said that if a serf escaped to a town and was able to live there for "a year and a day" he would automatically be free. So many ran away from the farms to the towns. This was made easier during outbreaks of plague and other diseases when so many died that there were few left to keep an eye on slaves.