When it comes right down to it, bikes have gears to allow your pedal speed (your cadence) to stay relatively steady and at about the same level of effort, regardless of if you're going down a big hill or up it. Your speed may change, but having gears means you can climb without killing yourself as well as to go fast on a descent, your pedaling still pushing the bike forward and not just twirling futilely, your feet unable to keep up with the speed of your wheels.
It works like this. If all the riding you ever did was on a flat level road at a constant speed, you wouldn't need gears at all. Your bike would have just one gear, set at just that right spot where where you can keep pedaling at a nice comfortable pace without killing yourself. From the riding you've done so far, you certainly know the feeling when you are cruising along in the cadence that is just right for you - going along at a steady clip but not straining yourself. That's what we're trying to have happen with our pedaling all the time.
But the world is not flat, as you know, and there are hills that we go up and down when we ride. Putting gears on the bike allows you to keep pedaling at that sweet point we talked about where you're most comfortable, regardless of the incline.
The tradeoff is that easier pedaling up a hill means you're going to be going slower. And to be able to zoom fast down a hill, you ride in a gear that is much higher than you'd ordinarily be able to start out in or even use to ride on the flat. In both cases your effort pushing on the pedals feels about the same, because the gearing makes adjustments that allow you to either climb easier or go faster than you would if you had just one gear.
