If you fail the vision test for the DMV you will not be able to renew your drivers license and will need to see an eye doctor.
You can come back later and renew your drivers license and take the eye test again after seeing an eye doctor and correcting your vision.
The DMV can also give you a form that the eye doctor can sign stating your vision is good enough for driving even when you don't pass the DMV eye tests.
In some cases a person may not pass the DMV eye test but may pass the eye doctor eye test.
In other cases a person who fails a DMV eye test may need eyeglasses, contact lenses or eye surgery to correct their vision.
If your drivers license is still valid you can still legally drive even when you fail the DMV vision test.
But until you get the vision corrected and pass the eye test you won't be able to get a new drivers license.
Seeing an eye doctor and renewing your drivers license as far in advance as you can do so is a good idea.
If your drivers license expires and then you go to renew it again then you may need to take the written and even driving tests again.