It means the sunglasses lens can block all UV rays including UVA and UVB which is invisible but can penetrate the clouds and cause a sunburn. All sunlight up to 400 nm wavelength can be blocked by these kinds of sunglasses lenses, so you can get a full protection.
UV 400 is a form of ultraviolet radiation protection associated with sunglasses. UV 400 protection is considered to provide close to 100 percent protection against harmful ultraviolet rays.of UVA and UVB ultraviolet radiation. It is called UV 400 because it blocks all rays of light up to a wavelength of 400 nanometers. It is considered the highest form of protection that lenses can provide. UV 400 has a higher level of protection than the European standard of protection, known as EN 1836:2005. This European standard allows for up to 5 percent of 380-nanometer rays to be transmitted through sunglasses.
Some sunglasses have a label saying "UV400", refers to the degree of protection of the lens. Sunglasses with this label insure that the lens can protect your eyes from all harmful UV rays that are under 400 nm, so they can give you a total protection from all ultraviolet harmful radiations.