Generally the clutch you find in a car is made up of three core components, a clutch disc, pressure plate and release bearing. At its most basic level, the clutch is used to transfer the rotation of the engine into rotation of the wheels.
When the car is turned on, the engine spins continuously and if the clutch pedal inside the car is depressed the clutch disc is pinned against the engine (specifically the engine’s flywheel). When the clutch pedal is pressed a fork inside the gearbox moves the release bearing against the pressure plate which releases the clutch disc allowing you to change gear. When engaged the clutch spin will transfer through the gearbox, onto the drive-shafts and eventually turn the wheels.
The friction material on the clutch disc wears down over time and the clutch will start to slip. The “slipping” is caused by the disc’s material wearing down and the inability to grip the flywheel. When the car is driving the engine speed (RPM) will increase rapidly but the car will accelerate very slowly. Unfortunately there is no simple solution for this and the clutch will have to be changed. Generally it is recommended to purchase a full clutch kit and replace all three of the core components.