Thank you for the opportunity to help you with your question!
When the driver manually selects at which gear (depending on speed)
should the car run, and he/ she changes the gears using a clutch, it is a
manual transmission. Clutch is the part which connects the engine to
transmission, when driver needs a gear change, he/she presses clutch
pedal/lever, which disconnects the wheels and engine, he/she changes the
gear and releases the clutch which puts the wheel back in connection
with the engine. With changed gear, the speed range of the car changes.
This is why when we go faster, we put higher gears. The reason behind
this is that engine has only a limited speed range.
Please let me know if you need any clarification. I'm always happy to answer your questions.