On the front should be a painting and the back it should say something like FB St Augustine Rocks (or whatever city it was painted in). The FB is for Facebook. It usually says something like please post pic on FB St. Augustine Rocks then keep or re-hide. Sometimes it just says #St.Aug if its a small rock.
What you're supposed to do is post a pic of it to the Facebook page mentioned on the back and where you found it so the owner knows it was found. Then you can keep it or re-hide it. Some people are watching their rocks found all over the world as they travel around and are moved by people on vacation who then take the to another town or country to re-hide the.
Some rocks are found without writing on back. This is usually done because the person just wants to make you smile and they don't care to know about who finds it or where it goes.
You can read more about how it started here :
http://thekindnessrocksproject.com/