First in an upcoming series. 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 (.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 of this coin as an immortal reminder of the key component to the guideline (well... as of the minting of the coin... guidelines evolve regularly - don't forget to read them before you click the "I read them" checkmark and submit that next cache page!) ;)
For the more studious of you out there, you can view the full text of the guidelines at this web page:
http://www.geocaching.com/about/guidelines.aspx