UPF stands for ultraviolet protection factor. It is a measure of how much UV passes through a fabric, in the way SPF is a measure for sunscreen.
The numbers
UPF 15 lets through about one fifteenth of UV. UPF 30 lets through about 3%. UPF 50+ lets through less than 2%. A thin white cotton T-shirt is usually around UPF 5, and less when wet.
What changes it
Tight weaves, darker colours and synthetic fibres block more. Stretching a fabric, wetting it and wearing it out all block less. We test our swim fabric wet and stretched, because that is how it is worn.
UPF only protects the skin the fabric covers.
So do I still need sunscreen?
Yes, on everything the suit does not cover, and reapplied after swimming. A wide hat and some shade at noon do more than any label.