Descriptions that begin with "1/8" means that the wheel/tire is used with a 1/8" axle.
The second number is usually the diameter.
Rears are generally around .800" wide
Example - A 1/8 x 13/16 Rear mounts on a 1/8" axle and is 13/16" in diameter.
Fronts are usually narrower and may include that info in the description
ProTracks of identical size that are $1 more are sticky natural rubber versions