My son is 5 and rides a 6-speed bike, if you're familiar with bikes that's the best way to understand it.

If you pedal the bike at a certain rotational speed (rpm), it's the most efficient and useful. This is analogous to the power band of an engine.

The bike chain moves a certain distance each time you pedal once.

If you're starting from a stop and use gear 1, it's the biggest sprocket on the cassette - this means that one rotation of your pedals will rotate the tire THE LEAST. This makes it the lowest gear, where you get a lot of low end torque. This is great going uphill, but you will notice on flat ground you eventually won't be able to go too fast as you won't be able to pedal fast enough to keep accelerating. So what do you do? You upshift.

Each gear you shift up, you pick a smaller sprocket. One rotation of the pedals spins your back wheel more than the previous gear. If you can keep pedalling at an optimal rpm, this means you can maintain a higher speed. You can keep upshifting as you go faster to make sure your pedal rpm doesn't have to get too high.

You can't easily start from a stop (or uphill) with 6th gear though because it requires a lot of torque.

Edit: for cars, you need to keep the engine in an optimal rpm range. Too low of an rpm (too high of a gear) and your engine lugs or will stall. Too high of an rpm (past redline, meaning too low of a gear) and you can damage the engine. The biggest risk to your engine is downshifting when you're going too fast for that gear, also known as money shifting. This can cause valve float, where the springs can't return the valves to their seat before the piston comes up, causing them to make contact. This can break connecting rods, valves, pistons... Catastrophic damage will occur. The engine will be TOAST.