Frag Collars are a novel solution to a common issue. You know how it is...you purchase a beautiful coral frag but it's stuck to a ceramic plug. In many cases, you have no option but to glue that plug (or plugs) to your live rock. In time the coral should grow over the plug but until then...your live rock looks like a porcupine. Frag Collars are meant to help blend or hide your ceramic plugs into the live rock. Simply cut the stem off your plug and place (I also recommend you glue) the frag plug in the collar and glue the collar anywhere you like.
A few additional benefits:
1. Frag Collars sink slowly. If dropped or one gets away from you while trying to place, it won't quickly fall behind rockwork where you can't reach it.
2. Frag Collars provide more surface area to affix to your rock work. In many cases, you can position your coral on the live rock where you could never have glued just the plug alone.
3. Frag Collars give your corals more surface to grow (I.e. zoas) so you can still reposition them before they are permanently stuck to the live rock...and again, while not looking as ugly as a huge ceramic plug!