RNG = Random Number Generator. UO/games use it to determine what loot is going to be given. Think about the old dungeon and dragon games back in the day, rolling to see what the loot will be. From what I have read, they used some college kids random number generator when they made UO and it was not very good. Think about crafting as an example, when you have a 60% chance to craft something and you fail like 30 times in a row when 60% is 6/10 so you should never fail 30 times in a row with a good RNG. Every time you snag a serpent, the RNG is used to determine if you get an MIB, Treasure Map, or zilch.
Yes I love fishing and treasure hunting. It is harder to use scripts for treasure hunting but a lot easier with fishing!