When you accelerate a vehicle, you turn your stored energy (gas in an ICE, battery charge in an EV) into kinetic energy. To slow the car down, you need to do something with this energy.

The common method is to use friction to release the energy as heat. The brake pads press on the brake rotor, and the energy is lost as heat.

Another way is to turn your electric motor as an electric generator, and recapture some of your kinetic energy as electricity. This can put the energy back in your battery to be used to accelerate again.

When you are running your vehicle in a race situation, where lots of hard accelerations are followed by hard decelerations, recapturing that energy can be a big advantage.