SPF stands for “sun protection factor,” and it’s a measure of how well sunscreen protects your skin. SPF refers to the length of time that sunscreen protects the skin. Suppose your skin naturally starts to burn in 20 minutes. If you use sunscreen with an SPF of 15, you won’t begin to burn for 5 hours–15 times longer. Always use a sunscreen with an SPF of 15 or higher.