This message is written on the inside of the locket, which is one of Voldemort's Horcruxes. It's a riddle, and its true meaning is only revealed later in the book.
The solution to the riddle is that the locket only opens when the person holding it is at their "close," meaning their end or death. This is because the locket is a Horcrux, and the Horcrux is destroyed along with the person holding it when they die.