Chainrings are the front set of rings with teeth on the chainset which the chain goes round. They are manufactured with a set number of teeth and the number of teeth on the chainring influences the amount of effort needed to push the chain round the chainset and propel the bike forward. The lower the number of teeth the less effort is required, easier to pedal, while a larger number of teeth increases the amount of effort needed, harder to pedal.
Chainrings are attached to the crank arms by 3, 4 or 5 bolts and diameter of the bolt pattern on the chainset determining which size of chainrings you can use. They are designed as either an inner (small), middle or outer (large) due to their placement on the chainset as inner, middle or outer ring. Newer double systems have a specific fitting outer ring.
If running a single chainring, make sure it is a dedicated single ring, ie for either DH or trail riding, and chain guide or bash guard compatible.