Ill assume that you are young and now you have learned the #1 rule in cycling.....It means that you have a bike not even suitable for a landfill. AND it Means you have an 18 speed "bike" and your 1-3 change the degree of difficulty in pedaling, 1 is on the smallest ring in front and 3 is the largest. One thru 6 are on the rear and determine the degree of difficulty in pedaling in 1-3. Ride and change them while you pedal only! Do it quick because the bike wont last long enough to really get the hang of it! These are also called "shifters" and more exact, Grip shifters since it is a nearly obsolete 6 speed.