A bump stop is a compressible spacer on the shock shaft that the suspension runs into near the end of its travel, acting as a second, progressive spring that stiffens that corner and limits how far it compresses.
Grassroots Motorsports describes them as “pieces of rubber or similarly soft material that act as progressive-rate springs” floating on the damper shaft. On a race car they’re not a crash pad, they’re a tuning tool. HP Academy explains that a bump stop you engage on purpose becomes “another spring operating in parallel with the main spring,” which lets you “run a softer main spring than we would otherwise be able to” — soft over the bumps, stiff once the car takes a set. The tuning happens at the gap: packers or spacers of different thickness set exactly where the stop picks up. Rules vary, so check whether your class allows them and how they get measured in tech.
A bump stop lets you run a soft spring for grip without letting the car bottom out, but the wrong gap turns a tuning aid into an unpredictable spring nobody logged.