A mana farm is a group of gems dedicated to gathering mana through the effect of orange gems, which "steals" mana from monsters.
While there are many types of mana farms, the most popular is the R/O trap farm.
The pure O trap farm can be used at any wizard level, but does not gather that much mana.
The R/O farm requires a maxed Red mastery for best results, and is the first to produce nearly unlimited amounts of mana.
The R/O/X is designed to gather the most mana/trackslot, while still controlling the monsters. needs to be at grade 11+ to work properly.
Prismatic farms are designed to do everything at once: halt the monsters, kill the monsters (eventually), steal from the monsters, and create the maximum amount of lag. They are not the best.
Whatever kind of mana farm is being used, they work best if the monsters are angered a bit, so that there are more monsters to go through the traps, and so that the monsters don't die too quickly.