The first two numbers represent the scopes magnification. The last number represents the scopes objective lense (farthest lense from the eye) size in milimeters.

A 3-9X40 would make an object to appear 3 times to 9 times larger than actual size. The larger the objective lense is the more light it will permit to enter your eye all other things being equal.

Many folks will place a scope of 1.5-5X or a 2-7X if they are expecting only short range shots. On the oposite end folks might jump up to a 4-12X or a 4-16X if they are hunting in very open terrain, such as would be the case for western pronghorn and plains mule deer. A 3-9X scope is a good all around magnification to use where you plan on having close in as well as farther shots.