Hi Cheri,
It is a "treasure hunt" using your GPS. (see this site for some information: Geocaching > Frequently Asked Questions)
My sister is very involved in it, but I've never tried it. People hide a "Cache" somewhere, then give GPS clues/directions on the internet. When you find the treasure, you can take a picture and post it to prove you were successful, but you always leave it for others to share in the adventure. My sister says the Cache is never valuable...the hunt and the adventure are the real treasure .
The site listed above is only one of many you'll find if you google it.