But do you know what that number next to SPF really means?

SPF indicates the level of protection the sunscreen provides against UVB rays, which are primarily responsible for causing sunburns. It actually has nothing to do with the temperature (SPF 40 does not mean it protects you from 40C heat). The number denotes the amount of time it would take for your skin to redden after applying the sunscreen compared to the time it would take without any sunscreen.

For example, if it normally takes 10 minutes for your skin to redden, using SPF 30 sunscreen theoretically provides 30 times the protection, allowing you to stay in the sun for 300 minutes (10 minutes x 30 SPF)