Changing the size of your motorcycle sprockets, either in the front and/or the rear, will change the way the bike feels when you ride.
You probably already know what the tranmission gears on your bike do. First gear gives you the most torgue (feels like pulling power) which is what you need when you're starting from a stop or slowly climbing a hill. Your top gear (maybe 4th or 5th or 6th depending on your bike) has less pull but more speed.
Changing the sizes of the sprockets on your bike can affect the overall gearing of the bike, making first gear even lower or your top gear even higher.
Installing a larger rear sprocket will make 1st gear seem more stronger and more powerful, will give you more acceleration, but you'll need to shift into 2nd gear sooner to keep the engive from revving. If you do mostly trail riding, a lreager rear sprocket will give you more power to climb steep hills. If you do mostly highway riding, decreasing the rear sprocket size will help give you a faster top speed.
Motocycle manufactures install sprockets suited for the intended use of the motorcycle, but they're trying to please everyone. So the sprocket ratio that came with your bike might not be ideal for you. If you want to change it slightly, to give you a little more power for hills for instance, you can easily change one or both sprockets.
A new rear sprocket usually costs more than a front sprocket, beacuse it's takes more steel/aluminum and work to make. But it's often easier to change a rear sprocket than a front one, and the effect of one extra tooth on the rear is less drastic than a one extra tooth in the front. Also, there's usually less room for a larger front sprocket in the front.