Here's a good definition of PRL from
http://www.phonescoop.com/glossary/term.php?gid=11
PRL
(Preferred Roaming List)
The PRL is a list of information that resides in the memory of a digital phone. It lists the frequency bands the phone can use in various parts of the country. (The smaller bands within Cellular or PCS, technically called blocks.)
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.
For example, say you had a Sprint PCS phone, and were travelling in an area with no Sprint coverage, weak Verizon coverage, and strong Qwest coverage. The PRL would tell the phone to look for towers using Sprint's band for that area. Finding none, it might tell the phone to search for towers in Verizon's band next, perhaps because Sprint's roaming agreement with Qwest was not as favorable, or none existed. The phone would use Verizon's towers, as dictated by the PRL, even though Qwest might provide a better signal.
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.