A feeder ring is a small ring that goes on top of the water so you can put food with in the ring and keep the food in the same spot without the filter blowing it all around. These are used mainly for bettas I believe because they are so slower moving and aren't able to chase food down like most fish.
I find it really useful for the betta yeah it keeps all the food together and my bettas have learned to hover under the feeding ring when I open the top of the tank so they can grab their bites before the black skirts come wizzing in.