If you see 400UV on the label of the sunglasses or descriptions, it means that the sunglasses that you choose can block light rays that have a wavelength of up to 400 nanometers. This kind of sunglasses usually protects you from 99-100 % of UVA and UVB light. They can protect your eyes from the UV damages. You should choose this kind of sunglasses.
400UV means the wavelength of the UV rays. The sunglasses with 400UV can protect your eyes from most of the harmful UV rays. Usually the seller will recommend customers to purchase this kind of sunglasses. But the sunglasses of other quality may be a little cheap so some people won't buy this kind of sunglasses. But I think if we want to protect our eyes from the UV rays, we need to buy a pair of good sunglasses. Hope this helped!