Cooking rings are rings varying in size from about 7 cm to 9 cm in diameter (2¾" to 3½"). They are commonly made from a non-stick, heat-resistant material such as silicone rubber, Teflon coated metals or stainless steel. They are used to prevent food from spreading whilst being cooked, e.g. crumpets, layered vegetables, rosti, eggs, mousses and desserts for example.
The advantage with silicone rubber is that it does not need to be greased to retain its non-stick properties.
Another use is to improve your plate presentation skills. Simply place the ring on the dish, then spoon in (for example) rice, mashed potato, vegetables, ice cream, fruit, etc. Compress slightly if necessary and level off with a spatula or, if you're really clever, with a sharp knife/palette knife to remove the excess, then swiftly remove the ring in a direct upwards direction.
Experiment by putting layers of foods with different textures/colours/particle sizes into the same ring (eg saffron rice, then peas, followed by mashed potato). It is even possible to layer one foodstuff on top of another by using successively smaller rings (for example, one big ring of swede mash, a middle size ring of pea puree, topped off with a tiny ring of tomato sugo/ragu).
Don't worry if some foods spread out a little bit after you lift the ring, although sometimes a bit more compression/squeezing into the ring can solve this. Even if this does happen, the end effect is slightly more pleasing than just plonking a splodge from a serving spoon/ladle of something which then is lying next to your wonderful Cookipedia-inspired masterpiece.