Riding the clutch is what learner drivers do to hold the car on a slight hill because they cannot use the handbrake,accel and clutch at the same time.
To slip the clutch is engage the clutch gradually but quickly. This is so the car has some momentum before stepping off the clutch completely. Allows you to get off the line without errupting into wheelspin.
It takes practise to get it right,too much clutch and you will either bog down or spin the wheels. Not enough and your clutch plate will just grind on the flywheel,resulting in a slow launch and a flywheel with heat spots in it.
1-2 seconds of clutch slip is what you want to aim for.Depending on your setup.