Ah, bloddy. It all makes sense . I understand your previous statements now.
Here's the abridged version of geocaching. Someone takes a waterproof box, and puts a piece of paper and a pencil in it. Then, they hide it in their favorite place, i.e. a park, monument, woods, etc. A place that the person would love to share with other people that love the outdoors.
Then, they note the location on their GPS and post those on the website. So, I go online, punch in my zip, and it gives me all of these boxes within a certain distance. I then use my GPS to find the locations. After getting close, you have to hunt a bit because they are hidden really well. You find it, sign the log book, then put it back.
Since geocachers love the outdoors and nature, they have dubbed a special event coming up. On that day, geocachers will all go to their favorite areas, and clean them up while/before/after doing the hunt. This is a regular practice, but somebody decided it would be nice PR to make an official event out of it.