A 21x10x9 is a rear tire, evident by the fact it's 10" wide:
21" tall top to bottom
9" inside diameter
The last number of this designation (9 here) must always match the first number of the rim size, ie 9x8.
OEM front tire is a 21x7x10 I believe. OEM rear is 20x10x9. For a front tire you can use anything that will fit on a 10" diameter and 5 or 5.5" wide rim. So you could have a 19x6x10 front, or a 23x8x10 front, or most sizes in between and a few bigger and smaller. But whatever you order, the last number has to be a 10 to match your rim diameter, and the width must be around 8" or less to fit on the 5 or 5.5" wide rim.
* 10" diameter x 5" wide wheel.
* 3+2 is the offset which means 3" of the total 5" width is on the inside of the hub and 2" is on the outside of the hub.
* Typically the stock front offset on most quads is 3+2.
* A 2+3 wheel will sit wider on the bike than a 3+2.