Sun Protection Factor, or SPF, is a measure of how well a sunscreen protects the skin from harmful ultraviolet (UV) rays, specifically UVB rays that cause sunburn. The SPF number indicates the level of protection; for example, an SPF of 30 means it would take 30 times longer for the skin to burn compared to not wearing any sunscreen. It's important to choose a sunscreen with an appropriate SPF for your skin type and the amount of sun exposure you expect. Additionally, reapplying sunscreen every two hours, or more often if swimming or sweating, is essential for maintaining effective protection against sun damage.