If you haven't heard of it, geocaching is a game or hobby where people hide a "cache" (usually a box with a paper log book and some "swag" or "trinkets" in it) in a public area for people to find. It's sort of like a scavenger hunt. Once hidden, the cache stays there for others to find. When someone discovers it, they are supposed sign the log book and hide it back in the same place.
The info about the "cache" is provided on a website (geocaching.com) that lists the item, it's GPS coordinates, a story behind it, and some hints to find it. People use the coordinates and the hints in the story to find the cache. They sign the log and then sometimes trade a trinket in the box for a more valuable one: something like a matchbox car, a wooden nickel, or a plastic bracelet, etc.
More info here:
Geocaching guide / Geocaching 101
There is also certain words and terms that geocachers use while writung about their finds (see the glossary http://www.geocaching.com/about/glossary.aspx). For example, they call non-geocachers "muggles." A geocacher might commonly be heard to say that while finding a cache we need to operate in "stealth" so that "muggles" don't see us.