When you let out the clutch pedal in your manual transmission car, the clutch disc (attached to the transmission) is pressed against the flywheel (attached to the engine) by the spring-loaded pressure plate. It's the friction from this clamping force that transfers torque from the engine through the transmission to the wheels.

Since clutches tranfer torque via friction, every time they're used they wear out a tiny bit, and once they wear out past a certain point they're no longer able to transfer torque. When your clutch is worn out like this, it can manifest in the clutch smoking/smelling when in use, or in the clutch "slipping" (engine RPMs increasing with no increase in car speed) in higher gears. In extreme cases, your clutch may slip in ALL gears, to the point where the car doesn't move even from a stop.

This clutch and pressure plate kit is made by Valeo, the Original Equipment Manufacturer for this particular Mini clutch. The only thing you're missing out on here compared to a Genuine Mini clutch kit bought from a dealer is the Mini box.

This kit comes with a release/throwout bearing, but does not come with pressure-plate-to-flywheel bolts, which should also be replaced when replacing the pressure plate.