Man years ago, on a remote place called "Dartmoor" there was the idea of leaving a small stamp or similar that you could collect and this was called a "letter box" You found them using a map and compass
Then as GPS became cheaper and more popular the "Letterboxing" became more diverse with caches over wider areas and sought using GPS and a map. Again the "prize" was often a small rubber stamp, you just left a message,or a small token, or exchanged for one already there. THis was called "Geocaching"
This further developed with technology when some companies cut out the need for a physical cache by using tags with a bar code. This meant that you had a record of your visit, but also via the app, your visit was also recorded at the site
Then came the travel bug, that has two functions. It can ne used as a stationary cahe as above, but can also be moved, and its journey can be tracked. You can also add stories to the site.
This is really just another extension to an older and developing hobby