A slipper clutch is an adjustable friction clutch between an RC car's spur gear and the drivetrain, tensioned by a spring and nut, that lets the spur slip under sudden load instead of shocking the gears and tires.
Scott Guyatt’s R/C Handbook puts it plainly: the slipper “is designed to slip a little before transmitting that entire horsepower to your overstressed rear tires.” How much slip is where people disagree. The RC10B6.4 manual says adjust in 1/8-turn increments “until you hear a faint slipping sound for 1-2 feet on takeoffs”; the Handbook says half a turn at a time “until the clutch only slips for about one meter.” CompetitionX skips distance, setting it so the front end “ALMOST lifts off the table” with the rear tires held. Surface moves it too: the Handbook says the slipper helps when the track is slippery or rough, and wants no slip on acceleration in high grip. CompetitionX calls it off-road-only and says slippers are illegal on-road.
The slipper is both a fuse for your drivetrain and a traction tool, so set it too tight and it stops protecting anything, too loose and you give away drive off the corner.