Clutch
The clutch disconnects the engine from the gearbox so a gear can be changed and the vehicle can move off smoothly from rest.
Press it fully to change gear and release it progressively. Slipping it - holding it part-way while driving - wears it out and is what produces the burning smell.
Do not rest your foot on the clutch pedal while driving, and do not hold the vehicle on a slope with the clutch. That is what the handbrake is for.
A clutch that bites very high, slips under load or judders as it engages is worn and needs attention before it strands you.