Corneal abrasions are very common.
A corneal abrasion occurs when something scratches the cornea. Common causes are fingernails (many parents come in with a fingernail scratch on their eye from a baby), foreign object in eye (such as a piece of grass, dust, glitter, hair), or something caught under the upper eyelid.
The top layer of the cornea called the epithelium is affected. You can see this with the area of the cornea that stains yellow.
The yellow area means that the epithelial cells on this area are damaged or dead. Usually the patient is in extreme eye pain with a watery eye, sensitivity to light, and a red eye.
The typical treatment to this is a bandage contact lens or an amniotic membrane (if severe enough), along with topical antibiotics. The good news is that the epithelial cells grow back fairly quickly. The eye regenerates faster than most other organs in the body.