UPF (Ultraviolet Protection Factor) is a standard used to measure the effectiveness of sun protective fabrics. This rate indicates how much of the sun's UV radiation is absorbed by the fabric. So if the fabric has a UPF rating of 50, the fabric only allows 1/50th of the sun's UV radiation to pass through the fabric. This means that the fabric will reduce the skins exposure to radiation by 50 times, which is equivalent to approximately 98% UV block. The big advantage of UPF over SPF (which is a standard measure to indicate how much time a person can be exposed to the sun before getting burned) is that the UPF standard measures protection from both UVA and UVB rays.
Exposure to the sun's harmful UV rays increases cancer risk and the Skin Cancer Foundation has noted that "suffering one or more blistering sunburns during childhood or adolescence more than doubles a persons chances of developing potentially deadly melanoma later in life". For more information please see this link.
It is important to remember that the fabric only provides protection for areas of the body that are covered. You should always remember to apply sunscreen to areas of the skin that remain exposed.
For the safety of your children's skin, Surlaplage swimwear fabrics are independently tested to ensure they provide UPF 50.