SPF is a short form for "Sun Protection Factor". SPF is represented by a number that measures how long skin covered with a certain sunscreen product will be protected. The SPF number is compared to how long (on average) it would take uncovered skin to burn.
Therefore, SPF is a measurement of the protection offered and not of the length of sun exposure time. However, many sunscreens with SPF will specify the amount of time you can stay in the sun without burning, based on the average skin tone. For instance, SPF 15 means it will take fifteen times longer to burn after the application on sunscreen.
SPF numbers range from 2 to as high as 60. When you use a sunscreen or sunblock, one with an SPF of 15 or higher is always ideal to choose. However, no study has shown that sun protection products with an SPF above 30 offer any more effectiveness than sunscreens with an SPF of 30.
Since SPF fends off burn times, it also prevents damage from UVB sunrays. However, sweating, swimming or other water activities will reduce the SPF protection, unless the product you're using is waterproof.
Apply sun protection with SPF before sun exposure. Spread the sun lotion or cream regularly and evenly across any exposed areas. If you have a light skin type that burns easily it's best to avoid the sun by using an umbrella or sitting under a tarp, however if you're forced to spend time in the sun, reapply sunscreen with SPF frequently to provide the skin with an invisible shield against the harmful sun rays.