The SPF – or “sun protection factor” – number on a sunscreen’s label is a guide to the product’s level of sunburn protection.
In general, the SPF number indicates how much longer you can stay exposed to the sun before getting sunburned when wearing sunscreen, as opposed to without sunscreen.
For example, it takes 15 times longer to burn with sunscreen SPF 15 than without sunscreen. However, whatever the SPF number, sunscreen should be reapplied at least every 2 hours after swimming or sweating, and towel drying.