SPF is short for Sun Protection Factor. It is the number printed on sunscreen — SPF 30, SPF 50 — telling you how much the cream protects your skin from burning in the sun. A bigger number means more protection. You say the three letters separately: ess-pee-eff. In real speech people use it as a noun on its own ("What SPF is that?", "I need a higher SPF") and also right before the number as a kind of label ("Put on SPF 50"). Many people now say SPF when they simply mean the sunscreen itself — "Did you bring the SPF?" — which dictionaries do not always show, but you will hear it, especially from younger speakers. A thing worth telling students: the number is about burning from one kind of sun ray only, which is why bottles add words like broad spectrum or, in Taiwan and Japan, PA+++. And a very high number is not as different as it looks — SPF 50 is only a little better than SPF 30, not almost twice as good. Careful writers keep it in capitals with no full stops.