Sunglasses that reflect or filter out 99-100% of UVA and UVB light, with wavelengths up to 400 nm, are UV400 protection on sun glasses. These kind of sun glasses are more protective than the usual ones, and they are better for our health. And as you know, there are many experts also suggesting that it is really very important to protect people' eyes by wearing sun glasses. I wish this could help you.
Yes, it means that you have a high level of protection from the sun rays. You 100 percent assured that you will not get hurt from the sun. UV400 means that your sun glasses are doing a really good job. UV rays includes UVA and UVB, which can not be seen but are very powerful to penetrate the clouds to shine on the land.