What does SPF stand for exactly?  SPF means sun protection factor.  It's a measure of how well a sunscreen shields your skin against ultraviolet B rays (UVB), which is the chief cause of sunburn.  Usually, the number is explained as the amount of time it takes an individual's skin to burn when it's covered in sunscreen compared with when it's not. 

For example, an SPF 30 would theoretically allow you to stay in the sun 30 times longer than you could without protection.  That means if you typically burn in 15 minutes, wearing an SPF 30 would extend that time to 7 1/2 hours. 

Here's the catch, though:  The level of SPF protection is calculated assuming you've applied the right amount of sunscreen.  For the average-sized person, that's an ounce (about 2 tablespoons, a blob the size of a golf ball) to adequately cover your face and body.  Most people apply about half that amount. 

Also, sweating and swimming decrease the amount of sunscreen on the skin over time.  For the best protection, apply sunscreen 15 to 30 minutes before going outdoors so that it has time to fully absorb into your skin.  Then-regardless of the SPF number on the bottle-reapply it every 2 hours, or immediately after you swim or perspire heavily. 

Don't rely on a tinge of burn on your skin as a sign you should reapply, either.  Sunburn often takes several hours after sun exposure to fully develop, so you could be burning even if your skin doesn't look red right away.