Basically it's for any post that's being discriminatory (as Azran Flame says), but isn't quite volatile enough to be considered trolling. So anyone who's post may be on topic but makes remarks in it that are condescending to a particular race, gender, sexuality, disability, etc.
For example, if you said "I don't understand" and someone replied along the lines of "Well of course not, you're a girl and girls never understand anything.", that would probably qualify for the infraction. If the post was "Oh my god, you stupid ****ing idiot, how can you not understand it's ****ing obvious isn't it, stupid *****!" then that would be trolling.
Not the best examples, but it's the dumbed down explanation.