"Named drivers" means that you have explicitly and contractually stated on the insurance who you want to be covered. Other than yourself, this should also include regular users of the car, for example, your spouse.
"Unnamed drivers" just refers to drivers who are not on it. They can still drive the car, but if they get your car in an accident, the insurer will cover significantly less of the damage.
You can always add more named drivers for maximum protection, but it will cost you more, especially if you name someone who is a young and inexperienced driver.