SPF stands for Sun Protection Factor, so the number alongside it will tell you how long the sun’s UV rays would take to redden or burn your skin compared to if you didn’t have any sunscreen on. So if you use an SPF of 45 properly (reapplying as often as needed), it would take your skin 45 times longer to burn than if you used no sunscreen at all.
The SPF value of sunscreen is important since it tells you a lot about how your skin will be protected for and how long. But a higher SPF doesn’t necessarily mean that you’ll have superior sun protection. It just means that your sun will stay protected for a more extended amount of time. That’s why opting for an SPF of at least 30 will provide you with ample time to reapply and keep your skin protected.