Sunglasses are a special kind of glasses that have been specially darkened to stop bright light from hurting your eyes. When you wear them, the world seems to become a little less bright, which can be a relief on very sunny days. They go all the way back to Roman times, and are especially popular today in the summer and on beaches.
However, shielding their eyes from light is not the only reason that people wear sunglasses. They can also sometimes look good in themselves as a fashion accessory, and people may also use them to hide their eyes from others when they are upset, or on drugs, or dont want people to see their eyes for some other reason celebrities who dont want their photo taken, for example. People with missing eyes or who are blind often wear sunglasses in preference to things like eyepatches.
One word of warning: many people will react badly towards you if you wear sunglasses indoors for no apparent reason, thinking that you are just doing it because you are a poser who wants to make a fashion statement. It can also be dangerous to wear sunglasses in places that arent very bright, as you wont be able to properly see where youre going and may trip over...