The white part of your eye.
It's just a part of your body, nothing special.
Examples:
The doctor looks at my eyes and says my scleras are healthy.
Her scleras are very white and bright.
I can see the scleras in the mirror.
The baby's big blue eyes and white scleras are so cute.
When you're tired, your scleras can look a little yellow.
The scleras are the part of your eye that you can see when you look at someone.