That is going through some areas multiple times, usually alone, to get lots of items, usually somewhere where the enemies are easy to beat with a certain build.
This can/was/is abused by bots, so they have been nerfing it.
Exposing my noobness here...but what in the name of all that's holy is nerfing? (Now that I know what farming is...because I'm not only new THIS game, I'm new to ALL games)