SPF (or sun protection factor) means how much longer you can stay in the sun without experiencing sun overexposure on your skin. By multiplying the SPF number by your personal self-protection time (the number of minutes you can be out before burning) you can work out how long it is safe for you to be in the sun and figure out the right SPF . e.g. if you can stay for 10 minutes in the sun without getting sunburned, a product with SPF 10 would multiply this “self-protection time” by 10 = 100 minutes).
Sun protection is also dependent on your own skin type. And above all, the sun protection is only given in full, if enough product is applied. Recommended are 2 mg/cm2 for one application, which means, for example, about 1 finger length of sun cream for the face or 2-3 tablespoons of product for an average adult’s body.