Someone hides a camouflaged container in a slightly interesting place (e.g. behind a statue, near an old church, some place with a nice view) and registers the GPS location, usually on an app. In the container they'll put a small (sometimes tiny) notebook and sometimes a little trinket of some sort. They may also put a couple of text hints on the app as to the container's exact location.

You then use the app to navigate to the GPS location and search for the container. Put your name/nickname in the notebook, take/swap/leave a trinket, and re hide the container.

We do it most often when we're on holiday with our kids (both under 10), it's usually a nice way to explore the area you're in, since the locations have generally been chosen by someone with a good knowledge of the local area. There's only been 1 so far we've been unable to get, and that's because it's 15m up a big tree and we don't have any climbing gear!