SPF stands for Sun Protection Factor, a number that tells you how well a sunscreen protects your skin from the sun's burning rays. When you see “SPF 30” on a sunscreen bottle, that number indicates how much longer you can stay in the sun without getting burned compared to wearing no sunscreen at all.

Here's how it works: if your unprotected skin would normally start turning pink after 10 minutes in the sun, SPF 30 sunscreen theoretically lets you stay out 30 times longer, or about 300 minutes (5 hours). However, in real life, you need to reapply sunscreen regularly because it wears off when you swim, sweat, or just move around.

The sun sends out invisible ultraviolet (UV) rays that can damage your skin, causing sunburn in the short term and more serious problems over many years. SPF measures protection specifically against UVB rays, the type that causes sunburn. Many dermatologists (doctors who specialize in skin) recommend using at least SPF 30, which blocks about 97% of UVB rays. Higher numbers like SPF 50 or 100 offer slightly more protection, but no sunscreen blocks 100% of the sun's rays.

You might also see sunscreens labeled “broad spectrum,” which means they protect against both UVB and UVA rays (another type of UV radiation).