Sun Protection Factor (SPF) is a measure of how well a sunscreen protects the skin from ultraviolet (UV) rays, specifically UVB rays that cause sunburn. The SPF number indicates the level of protection; for example, SPF 30 means it would take 30 times longer for the skin to burn compared to not wearing sunscreen. Using a higher SPF does not mean you can stay in the sun longer without reapplying. It is essential to apply sunscreen generously and reapply every two hours, or more often if swimming or sweating, to maintain effective protection against skin damage and skin cancer.