Teeth. That’s what all the figures refer to. You have your chainrings at the front – the rings up by the pedals – and the sprockets at the back – the ones attached to the hub of your rear wheel. And they all have different numbers of teeth.
Chances are that you have two chainrings. Maybe a standard set-up with something like 52 and 39 teeth (often written as 52-39T or 39/52T), or a compact, which usually has 50 and 34 teeth. The fewer chainring teeth, the lower (easier) the gear.
Then you’ll probably have somewhere between nine and 11 sprockets (maybe fewer) at the back, depending on the groupset fitted to your bike. So the cassette (the group of sprockets) might range from a 12-tooth small sprocket to a 25-tooth large sprocket. That would be described as a 12-25T cassette. The fewer teeth on the sprocket, the higher (harder) the gear.
It’s the ratio between the number of teeth on the chainring and the number of teeth on the sprocket that tells you how hard the gear is. If the chain is on the 39-tooth chainring and a 13-tooth sprocket (a 3:1 ratio), you’re in a bigger gear than if you’re in the 52-tooth chainring and a 21-tooth sprocket (roughly 2.5:1). In other words, you’ll go further with every crank revolution but it’s more difficult to push the pedals around.
There are no hard and fast rules about what gear ratio you should use at any particular time. The right choice comes down to many factors, including terrain, conditions, your strength and fitness, and how your body works.
Speaking of which, now is the ideal time to experiment with your gear selection and cadence. Spend some time trying out different approaches – small gears with a high cadence; big gears with a slower cadence – to work out what’s most comfortable and effective for you.