Sun Protection Factor (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, SPF 30 means it would take 30 times longer for the skin to burn compared to not wearing any sunscreen. It's important to apply sunscreen generously and reapply it every two hours, especially after swimming or sweating. Using a broad-spectrum sunscreen, which protects against both UVB and UVA rays, is recommended for comprehensive skin protection during outdoor activities.