What is tested
Most states test visual acuity (how clearly you see distant objects), peripheral vision (how wide your field of view is), and color recognition (the ability to distinguish red, yellow, and green for traffic signals). The standard acuity threshold for an unrestricted license is roughly 20/40 in your better eye.
Glasses and contact lenses
If you wear glasses or contact lenses, bring them to the test and use them for the screening. Passing the vision test only with corrective lenses adds a 'corrective lenses' restriction to your permit. Driving without your glasses or contacts when the restriction applies is a moving violation.
If you fail
Failing the screening usually means scheduling an eye exam with an optometrist or ophthalmologist. Bring the doctor's vision report back to the DMV; in most states, that is enough to issue the permit with whatever corrective restriction is needed.