Tire additive, also known as sauce or traction compound is a fluid that can be applied to tires to increase their traction by making them softer.
Additive is mostly used on smooth tracks or on-road tracks, as dust will stick to it and can actually decrease the grip of the tire.
Additive is widely used for off-road racing and almost always used for on-road racing. There are additives for both rubber and foam tires.
Additive is most often applied about 15 minutes before a race to give it time to absorb into the rubber (or foam) and take effect, but please check your additive's instructions, as some may vary. It can be applied in combination with tire warmers - the warmers will help the additive soften the tires even more than usual.