Let's start with the meaning of SPF:
SPF means “Sun Protection Factor”
The SPF number tells you how long the sun's UV radiation would take to save your skin. Ideally, with SPF 30 it would take you 30 times longer to burn than if you weren't wearing sunscreen. In essence SPF efficiency is a measure of the percentage of harmful UV rays it block from being absorbed by your skin.
In the image below you can see how sun filters protect our skin: