SPF stands for sun protection factor. The SPF number is a measurement of the amount of UVB protection a sunscreen offers. SPF is not an indication of how much time you can spend in the sun. If you use a sunscreen with an SPF of 30 rather than one with an SPF of 15, it doesn't mean you can stay in the sun twice as long. Rather, it means that a sunscreen with an SPF of 15 filters out about 93 percent of UVB rays. A sunscreen with an SPF of 30 offers slightly more protection because it filters about 97 percent of UVB rays.