What is SPF

SPF measures sunscreen protection from UVB rays it means 
"Sun Protection Factor" it indicates how many multiples
of time you're safe to be out the sun before you burn.

For example if it take 20 minutes for your unprotected skin to start turning red ,using an SPF 15 sunscreen theoretically prevents reddening 15 times longer- about five hours SPF 15 will give you 15 times the number of minutes before you burn that you'd have with no sunscreen protection at all.