SPF stands for sun protection factor. It is a measure of ultraviolet B protection. It is the amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in non sunscreen-protected skin.
For example, a sunscreen with an SPF of 15 will delay the onset of a sunburn in a person who would otherwise burn in 10 minutes to burn in 150 minutes. So SPF 15 sunscreen allows a person to stay out in the sun 15 times longer. It is important to remember that SPF does not predict ultraviolet A protection.