Your bike is equipped with gears, depending on how much you paid for it and when it was new somewhere between 10 and 12, If your current steed is a little long in the tooth you could be sporting a 9 speed or heaven forbid even less.
You can usually tell how many you have by a sticker somewhere proudly proclaiming the number of gears. In this modern age you could be forgiven for thinking this is just another marketing scheme to sell more bike (well it kinda is) just like to 20grams this years model saved over the last one but and this is a bit but, more gears is a genuine benefit to you.
I'm not going to get into the single front ring movement or the application of, benefits and limitations of triple front rings, that's for another time, today is all about the basics.
First up the human body has a pretty narrow operating window when it comes to pedal speed and even more narrow when we look at an individual's peak efficiency. The main purpose of the gearing system is to keep you in this window.
The most common set up in road cycling currently is a two ring crankset (the bit at your pedals) and 10 or 11 speeds at the cassette or the wheel, for this exercise the number isn't important.
What is important is to understand how the mechanical advantage of gearing works.
Very basically if you have a big gear (at the crankset) running to a small gear (at the cassette) you get a change in the speed at which the driven gear turns (the cassette or effectively the wheel) relative to the input speed. For this change in speed we get a change in the torque needed to turn the cranks (and in turn the wheel). So if you have the big front ring and the smallest rear cassette gear you get the least mechanical advantage and have to deal with all the torque load via your muscles but the speed you move the bike to is greater within the limits of the pedal speed the body can make. The opposite of this example is the smaller front gear and the largest rear gear. This arrangement gives us a faster pedal speed for a slower wheel speed which on the surface of it seems pointless but and here is the important part, the reduction in torque at the pedals. The mechanical advantage effectively lowers the force need to turn the cranks with the trade off being the wheel speed is much slower.
Now at this point you should probably be seeing the benefit of the gearing system and there a lot more to talk about when it comes to the effect torque loads have on fatigue and muscle strength but again to keep this simple, maintaining you cadence (leg speed or RPM) in an efficient range ( again another rabbit hole to disappear down) you need to use your gears and to maintain momentum over rises and into climbs you need to get familiar with your gearing, be able to make a change before you need it so to keep momentum.
A few tips about gear selection
When your riding into a rise, depending on the grade being slightly ahead of the change ( a little early) or dropping two gears at once is the best technique, having a little more cadence will work better than getting behind. .