There are questions on this site tagged geocaching.
This appears to be some kind of GPS game. How is it played?
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In short, it's a high-tech treasure hunt.
geocaching.com contains listings for "geocaches" all over the world; containers that are hidden at particular co-ordinates (discoverable via GPS) and often with clues once you get to the rough location. You download the co-ords / clues onto your GPS, go to the spot, look for them and then sign a log in the cache when you find it (to show you've been there.)
They come in all shapes and sizes, everything from small magnetic containers about the size of a thimble to ammo cans and buckets (and in some cases, even bigger though those are rare.)
Sometimes you have to solve clues or wander around the area gathering pieces of information before you get the final co-ordinates, these are known as multi-caches and mystery caches. You can of course place a new geocache out yourself and then register that with the site so others can find it.
Sometimes caches are really easy to find (I've seen some just walking to the location) and others are horrendously difficult (I looked for a good hour or so for one in particular before finding it!) They have difficulty ratings though which should give you a rough idea. You get better at finding them as you go - start with easy ones and work your way up!
Simply put, a cache is hidden at a particular location, and clues are provided (usually via a website, but any media can work) so that searchers can find it.
The usual plan is then to open the cache, take an item from it and replace it with something you have brought, and then note your discovery either on a log in the cache or on the website.
Geocoins are often left in caches, and are trackable, giving people an opportunity to collect sets online before leaving the geocoin in another cache.
It is astonishingly popular the world over, and isn't as easy as it sounds - not just because clues can be very cryptic, and caches can be hidden really well, but because GPS units aren't always that accurate.
An alternative - which I really like - is the Degree Confluence Project
The goal of the project is to visit each of the latitude and longitude integer degree intersections in the world, and to take pictures at each location
So instead of finding a cache, you find the integer coordinates.