Excessive exposure to UV rays from the sun can cause premature aging, painful sunburns, and even increase one's risk of skin cancer. Fortunately, there are plenty of ways to prevent excessive sun exposure without retreating to the indoors such as applying sunscreen and wearing protective clothing.
UPF: Ultraviolet Protection Factor. A fabric's UPF rating is determined by how effectively a garment protects skin against UVA and UBA rays emitted from the sun. This is quantified and translated to the numerical rating often displayed on clothing labels.
What affects the UPF rating of a garment?
Type of fabric
Construction of the garment. For instance, fabrics made of tightly woven fibers provide more protection than loosely-woven fabrics.
For more information: UPF-Rated Clothing: What's the Deal?