This term is used when a fucking douche bag wears their bottoms around their fucking ass. Originally, low-riding whilst in jail meant that you were 'keen' for a 'butt fuck'. Low-riding is highly common amongst gay fuck tards.
Person 1: "I'm low-riding."
Person 2: "You're gay?"
Person 1: "Yeah."
To low ride is to wear your trousers(pants) low down, usually around the upper legs. Underpants are usually visible, but often long T-Shirts are used to hide it.
"Man, check out how low that guy is low riding!"
When one sits on the toilet with no seat down so as to sink into the toilet bowl as much as possible to have his/her buttocks closer to the water line, thus reducing the risk of a water splash-back incident (also known as a "fitzgerlad")
Guy #1: Hey man how did you go on the crapper the other night after that pie eating competition, did you end up with a Fitzgerald???
Guy #2: Na bro I just executed the lowriding position and all was well.
Guy #1: Genius!