(Preferred Roaming List)
The PRL is a list of information that resides in the memory of some kinds of digital phones. It lists the radio frequencies the phone can use in various geographic areas.
The part of the list for each area is ordered by the bands the phone should try to use first. Therefore it's a kind of priority list for which towers the phone should use.
The PRL helps determine which home-network towers to use, and also which towers belonging to other networks to use in roaming situations (areas where the home network has no coverage.) When roaming, the PRL may instruct the phone to use the network with the best roaming rate for the carrier, rather than the one with the strongest signal at the moment.
Since a PRL tells the phone "where" to search for a signal, as carrier networks change over time, an updated PRL may be required for a phone to "see" all of the coverage that it should, both with the home network and for roaming.