This coin serves as a fun reminder of one of the cardinal rules (er... guidelines) of Geocaching. Caches have to be spaced out and 528 ft (.1 miles) or 161 meters is the minimum distance you can place caches from each other.
The guidelines, of course, go into more detail about this requirement. You can read some of the text on the outer ring
Size: 54mm in diameter, 5mm thick
Trackable at geocaching.com with its own icon: