A sway bar is a wire bar that ties an RC car's left and right suspension arms together at one end of the chassis, resisting body roll in a corner. Thicker wire means more roll stiffness.
Bars are sold by wire diameter in tenths of a millimeter and usually color-coded — Team Associated’s RC10T6.1 rear set, for example, is 1.1mm white, 1.2mm gray and 1.3mm blue. Thicker bars cut chassis roll and suit high-grip, smooth surfaces; thinner or no bar lets the car lean and load the outside tires, which is what you want on low grip. Front and rear are separate levers: a thicker front bar reduces front grip and adds off-power steering on entry, while a thicker rear adds mid-corner stability and on-power steering. Full-size race cars use the same part for the same reason; RC racers just carry a set and swap the whole wire.
Roll stiffness decides whether the car leans enough to load the outside tires or trips over itself and traction-rolls, and a bar swap is one of the biggest handling changes you can make in five minutes.