Ultraviolet light damages the eye in the same way it damages skin. Over the years it raises the risk of cataracts, pterygium (a fleshy growth on the white of the eye) and changes at the back of the eye.
Choose sunglasses that block 100% of UV, wear them whenever you are outdoors, and add a wide-brimmed hat on very bright days. Children's eyes let in more UV than adults', so they need protection most of all.