In the US, it's generally a show that has had more than 100 episodes, and the rights are sold to a non-network station, which shows them at some off-time (like 6:30 pm, or midnight), usually every weekday.
For example, "Friends" is aired by NBC in the States. However, in my city, a local non-network station called WGN has the syndication rights, so they air older episodes (anything not in the current season) Monday through Friday at 7:00 pm and 11:00 pm.
What is your favorite children's book?