Stripping cars is a way of breaking down a car into fusion and stage 6 parts - but in doing so the car will be lost. This is usually only desirable if you have multiple versions of the same car, and you want to use the parts from one copy of a car to improve another copy of the same model.
To strip a car, select it in the garage then tap the info “(i)” button in the right corner of your game screen.
Tap Strip to start.
Remember that the parts you get from stripping a car aren’t related to the upgrades you’ve fitted to that car, so don't expect to get back all of stage 6 and fusion parts you've previously fitted.