I've discovered this recently - it's sort of like treasure hunting. People hide caches, and post their locations on a web site. You have to find the cache, and usually you write your name and date in the log provided.
I took my niece out for a geocaching session earlier this week - we found three caches near to us in about 1 1/2 hours. It was fun reading the clues, and following the directions on the tablet app to get to the required location.
Some of the hints require a bit of interpretation - one was "take a seat and Vue the post" - the casche was under a fence post in the direction of the Vue cinema!!
Anyway, it's good fun, and adds an extra element to a walk, or having a look around when you're wanting something to do for a few minutes.