Side bite is lateral grip at the rear of a dirt car — the rear tires' ability to hold against cornering load, as opposed to forward bite, which is traction in the direction the car is travelling.
Two explanations are in circulation and they don’t agree. The traditional one is mechanical: the car rolls onto the right rear and the tread blocks dig in, which Hyper Racing describes as working “like a paddle in the water.” Hyper Racing then argues that reading only holds “when the track is wet or slightly wet,” because once the surface goes dry-slick “the tire’s grooves can no longer work into the dirt, at least not much because the dirt is harder than the rubber.” Their replacement is lateral traction, a weight-transfer argument: “when you see a car rolling on the right rear, the car is tight because it is transferring less weight, not more weight.” Which reading applies depends on what the track is doing that night.
Confuse side bite with forward bite and you spend the night adjusting the wrong thing — one is about holding the corner, the other is about leaving it.