SPF (sun protection factor) indicates how much solar energy is required to burn the skin compared to without sunscreen. SPF 15 means it takes 15 times longer to burn than unprotected. However, other factors like solar energy strength also determine burn time. For beach trips, doctors recommend at most SPF 30 that is waterproof and sweatproof. Reapply every hour, especially after water, and wear a hat and sit in shade when possible. The document asks readers to share their best beach experience.