Shifting gears is how you make your bike easier or harder to pedal, and having that range can make cycling easier and more fun. Most modern adult bicycles have between 7 and 30 speeds. They arrive at that number by having rear sprockets with between 7 and 11 gears and up to three chainrings on the front. 3×11 drivetrains are unusual, to say the least, because of the shifting setups that are most common.
Having that number does not mean that there are that many unique speeds. It means there are that many total possible combinations, but some of the gear ratios will be duplicated or so close as to be unnoticeably different. Gear ratios are what make a certain gear harder or easier to pedal in; a dramatically bigger front chainring going to a small gear on the back will be much harder to pedal than the smallest chainring on the front going to a larger gear on the back. So being on the biggest chainring on the front and the smallest in the back is for going downhill with a tailwind, going as fast as possible, while being on the smallest in the front and the largest in the back is for making your way slowly up a steep hill. Those are, respectively, your highest and lowest gears.
Most of the time, though, you will not be at those extremes. The largest chainring on the front corresponds to your highest gears, the three smallest gears in the back. The middle chainring corresponds to the middle four or five gears on the back. The middle chainring is where most riders spend most of their time. The smallest chainring corresponds to your easiest gears, the largest three gears on the back. You want to keep your gears in the ones they correspond to. This is because your drive train was designed to function best for the longest time that way.
Using the gears in combinations like the largest gear in the front and the largest gear in the back is called crossing gears. It twists the chain, putting undue stress on both the chain and the gears. This stress wears everything out more quickly. Additionally, pairing the largest gear with the largest can stretch the chain and pairing the smallest with the smallest can cause a great deal of chain slop.
Other Shifting Configurations
Some bicycles have only two front chain rings. Those are most commonly road or triathlon bicycles, because two chainrings is lighter than three. There are also fewer gears to cross, while still providing a range to accommodate climbing. A rising number of mountain bikes have only rear gears. This is because it lets you keep your chain more firmly in place, though it cuts the gear range.
Some bikes have only a single speed, but those are primarily BMX and kid’s bikes. In BMX, the single speed is advantageous because there are fewer things to go wrong on a bad landing. Additionally, the single speed makes the bike lighter, which is also the primary benefit in a kids’ bike. The other benefit in a kids bike is that a single speed is simpler to learn for newer riders.