During Hiatus
Most episodic television shows run only during a certain season. Normally this season is from early September until late May. In the summer (and sometimes around the holidays) shows stop filming. No more episodes are being produced, so the network airs previous episodes in their stead.
Syndication
When a television program becomes popular, it often goes into syndication. This is when many episodes of the program are sold as a package for a large sum of money. Generally the buyer is either a [cable company]? or a host of local television stations. Often, programs are not economical until they are sold for syndication.