Bubble rings are rings of air underwater, much like smoke rings in air. (Both are examples of vortex rings.)

Bottlenose dolphins and Humpback whales (which also create bubble nets for hunting) are both known to blow bubble rings.

Learn to Blow Bubble Rings

Since you need to turn your face up parallel to the surface of the water, you should use nose plugs or pinch your nose shut. Wear a mask or goggles so as to see what you're doing.

Take a big breath so that you have air to work with. Situate yourself some feet under still water (the bottom of a pool ladder works well). Turn your face up towards the surface. Stick your tongue out and form an O around it with your lips. Blow out cleanly and a little 'explosively' while you pull your tongue in. You're attempting to blow a single bubble about the size of a ping-pong ball.

Practice until you get clean, unbroken rings. See how long you can make them last, and how large you can make them.

Once you've learned the feel of the blowing motion, you don't need to stick your tongue out to accomplish it.