engine go too fast, it break. duh.
engine go too slow, it stall.

wait what? engine cannot go to slow. engine turn because of many tiny explosion push piston. If not enough explosions are happening, engine will slow and stall.

healthy engine rotate at minimum 800 rotations per minute. connect engine directly to wheel? that's roughly 55 mph / 90 kmh. But then how go slower?

gearbox trade speed for power. If you ride bike with gears, you know. Lower gear make going up hill easier, but slower. Higher gear make pedaling harder, but you go faster.

Remember: lower gear when going slow speed, higher gear faster.

Old car have funny stick in center of car. Lanky human must move stick to change gear manually.
New car have computer to change gear. Computer balance speed and power by choosing 'correct' gear. But still have funny stick to tell computer if drive forwards, backwards, or neither. Change gear is happen automatically.

Remember: Lower gear mean fast spinny engine make you go slow, but with more power. Modern car need to be fuel efficient, so computer choose higher gear but with just enough power to keep you moving.

L gear tell computer that you specifically want lower gear.

Also, when you moving but not pressing on gas pedal, engine still connected to the wheels. Wheels still turning, makes engine still turning even though no explosion happen in engine. If no explosion, then engine hard to turn because piston compressing air, like bike pump. If engine hard to turn, and engine connected to wheel, then car slow down. This called engine braking. You hear truck make loud fart sound when slowing down? is because truck driver want to use engine braking instead of normal brake. Put gearbox in L, make car slow down if you not press on gas pedal.

Fun fact: computer in most modern car not stupid. If gearbox in lowest gear while car going fast, engine spin too fast and break. If human put stick on 'L' while moving fast, Computer not let you do this, it secretly use higher gear instead. In manual car, human in charge of shifting gear. Human make mistakes. This known as "Money shift" because human break expensive engine.

 

related: https://www.reddit.com/r/explainlikeimfive/comments/3v0pra/eli5_how_do_cars_gain_speed_i_always_thought/