From The TV IV
Booster is the term for a low power television station which "boosts" the signal of its parent television station by transmitting it to areas where the signal is weak. These stations are not allowed on the same frequency as their parent stations due to interference artifacts such as ghosting. Interference in general is a problem with booster stations, but they have been prevented by using atomic time to synchronize with their parent TV stations.
In the United States, booster stations have call letters which contain both letters and numbers, like KH129R. However, in Canada and Mexico, boosters are simply tacked on using a numeral suffix, such as CHBE-1.