best way I can explain it is your legs provide the energy to the crank, which turns the chain ring and then a cog on the rear cassette which turns the rear wheel.

When you start pedaling from a stop your human engine has an easier time turning the rear wheel at a higher RPM. Meaning you make more full pedal rotations to turn the rear wheel one time around. As you build up speed and increase RPM, you need to reduce the number of pedal RPMs to keep gaining speed. Among the many reasons this is required, one is that your human engine has a RPM limit at which point you are either transferring no energy to the rear wheel OR, you physically cannot maintain the very high RPM. The other is the momentum you are carrying. As you build up speed your gearing must increase to allow your legs the ability to put the energy into the rear wheel and then into ground. This is facilitated by reducing the number of pedal revolutions needed to turn the rear wheel one time around...

I'm not sure what I am saying in my head reads well... I'm more of a verbal teacher than a writer.