The phrase "less downforce" means that there is a decreased upward force pushing on an object, which reduces its stability and grip on the ground. Full definition
You feel the increased grip effects of the Performante's «aero - vectoring» technology over supercars with less downforce, such as the McLaren 720S, though it didn't come close to the aero - assassin grip of the Ford GT. (automobilemag.com)
Less drag makes it faster on the straights, but slightly less downforce means slightly lower speeds in certain corners. (wtf1.com)
This race car used a large rear wing that could be augmented to offer more or less downforce when needed. (automobilemag.com)