Geocaching is a type of treasure hunt done with the help of a GPS.
It consists of finding a container which was previously hidden by a person, in an outdoor place, and when you find it you will find a “treasure” (it can be anything) inside the container. You can take the treasure or part of it, but in return, you must hide something else of equal or higher value.
The name comes from two words: “geo” (as in Geography, Geology), from Greek, meaning ‘Earth’, and “cache”, from “cacher”, French for hide. The container is called geocache.
You can read more at the official geocaching page, where you can start a geocache hunt if you want.