Farming is a term used to describe repeated killing of the same group of monsters in the same area again and again in order to make money. It is often done solo so you don't have to split gold drops with the other 8 members of your party.
You can also farm for green weapons by repeatedly killing a boss over and over again.
There is an entire sub-forum dedicated to farming.
http://www.guildwarsguru.com/forum/f...play.php?f=234
Some of the stickies there explain popular farm locations and tactics for every class.