Okay, so gear inches is like a way to measure how easy or hard it is to pedal your bike. When you ride your bike, you turn your pedals and that turns the wheels. The bigger the wheel, the farther it goes with each turn.
But the thing is, your bike has gears that can make it easier or harder to turn the wheels. When you shift to a lower gear, it makes it easier to pedal but you won't go as fast. When you shift to a higher gear, it makes it harder to pedal but you'll go faster.
So gear inches is a way of measuring how easy or hard it is to pedal your bike with a certain gear. It's like a number that tells you how big your wheel would be if it didn't move when you turned the pedals.
For example, let's say you have a bike with 26-inch wheels and you shift to a gear that has 50 gear inches. That means it would be like you were pedaling a bike with 50-inch wheels. If you shifted to a gear with 20 gear inches, it would be like you were pedaling a bike with 20-inch wheels.
Basically, gear inches helps you figure out which gear to use depending on how easy or hard you want it to be to pedal your bike, especially if you have to go up or down a hill!