The "S" in "SPF" stands for "sun," since "SPF" is an acronym for 'sun protection factor." SPF essentially indicates the level of protection from ultraviolet B, also known as UVB rays.
How much protection a particular SPF provides depends on the user. For example, if someone who normally burns in 20 minutes applies an SPF 30 sunscreen, then that sunscreen will provide them with 30 times the protection that he'd have if he was wearing no sunscreen at all. In this example, the person wearing the sunscreen would have (20 minutes times 30) 600 minutes of time in the sun before he would start to burn.