-2 lenses” for sunglasses usually means the lenses have a -2.00 prescription power for nearsightedness (myopia).
So if someone wears -2 glasses, they can get:
• Prescription sunglasses with -2 lenses
or
• Regular sunglasses worn over contact lenses.
A -2 prescription is considered mild to moderate nearsightedness — you can see nearby things clearly, but far objects look blurry without glasses.