It's an outdoor game like a scavenger hunt. Other people place geocaches (tupperwares or similar containers) somewhere out in nature and mark the spot on an online map. Anyone can find them and sign the log book inside, sometimes there are other items to trade as well. Free to play at geocaching.com.
I've done this lots while travelling in Europe and North America, it's fantastic if you want to get away from typical tourist hotspots for a while. It's helped me discover so many spots I never would've seen advertised anywhere
That's so sweet! Sounds very much like geocaching