It's a community treasure hunting game. A person hides something, then posts GPS coordinates and/or clues to lead you there. Often the hidden item is a sealed container so people can leave log books or unique trinkets. Some of the trinkets have serial numbers you can use to see where they've traveled.

I haven't done it in years, but I remember it being a lot of fun. Hunting for caches led me to a bunch of interesting new places that I would never have explored otherwise.