In general, a lowrider usually means a vehicle (i.e. car/truck/SUV/etc.) that's been lowered significantly below its stock ride height. Usually, this is achieved by drop spindles, cut springs, etc..
More specifically, a lowrider is a car with hydraulics. Lowrider Style and Culture generally come from Southern California. Some smart Chicanos bought old WW2 aircraft and hacked their cars to support it =) Suddenly switch-adjustable, variable-ride-height pimp-mobiles were born :-)