"Motor racing is dangerous." That notation shows up these days on everything from seat belts and helmets to the catalogues that sell parts to racers. Solo racing has an outstanding safety record, with a strong and active Safety Steward program to enforce it. The Safety Steward is the one worker specialty in Solo racing requiring classroom training and a license.
Solo course design and event operation rules are written to keep everyone safe. We must maintain a certain distance from fixed objects, for example (although our East Crawford site basically has no fixed objects), and there is a maximum speed guideline. But we never say "never." Accidents can happen. Someone can just get it wrong. Injury or death can occur. Statistically you are safer driving an autocross than driving to the supermarket but there are no guarantees in life.
The waiver you sign does not mean you sign away all your rights but it does say you are aware of what is going on. Primarily it kicks in the insurance coverage. If you are injured, you are covered - it is secondary to your own health/accident policy, but has a multi-million dollar upper limit. If you hurt your car the event insurance does not cover that, but does cover SCCA's liability for any damage.
You participate at your own risk. The risk is very low but it is not nonexistent.