Box is not clear, tell me (SnowRunner)

Hi all. I generally far from the mechanics (both in terms of mechanical works on the car and manual transmission SUVs), so I do not understand - why the need for transmission H in the game. L it is clear, is reduced. And that's what the H? Switch to it, it displays a picture of the engine. Hit the gas, and the engine stalls.
In General, help explain to the new guy - what kind of strange bird is this? And HOW and, most importantly, WHERE to use it?

Box sled in the first Chevy.

Thanks in advance for the answer.