Okay, imagine you are riding a bicycle and you are going up a hill. Your legs may start to feel pretty tired, right? But what if you could change the bicycle gear to make it easier for you to pedal? That's what bicycle gearing is all about - it helps you change how hard or how easy it is to pedal.
A bicycle has a chain that connects two sets of gears - the ones on the pedals (the ones you push with your feet) and the ones on the wheel (the ones that make the bicycle move forward). To change the gear, you use the gears located near the pedals.
When you shift the gear, you are changing the size of the gears on both your pedals and your bike's wheels. The bigger gears are harder to pedal but make you go faster while the smaller gears are easier to pedal but make you go slower.
So, if you are going uphill, you would want to use the smaller gears to make pedaling easier. But if you are going downhill or on a flat surface, then you can use the bigger gears to go faster. By changing gears, you can adjust how hard or easy it is to pedal the bike, so you can comfortably ride it no matter what terrain you're on!