Sex bracelets are inexpensive and colorful jelly bracelets. They sometimes signal, based on their color, willingness to perform a certain sex act. They are sometimes used in games in which sexual favors are exchanged.
Apparently all the kids are wearing them. Here’s a the decoder chart which describes what colour means what sex act. If I was wearing a green one, I’d want to clarify its alternative meanings, which include both ‘outdoor sex’ (an homage to environmentalism, I gather) and ‘hug’. To me, the whole thing sounds like an overblown media myth.