Farming is a generic mmo term. In flyff it's most commonly used to refer to the repeated killing of monsters to obtain items or gold. So if someone says they're farming a giant, they mean they're killing a giant repeatedly for drops, or someone could farm for a specific item like say sunstones by killing dungeon mobs.
Those glowing fields you mentioned are Collector Fields. You can buy a collector and batteries, and use them to extract certain items from the fields. The
official wiki has a page on it that should explain it well enough.
Hope that helps some :)