SPF, or Sun Protection Factor, is a rating system that determines how long a sunscreen protects your skin from the sun's UVB rays. The SPF number is a measure of how much longer you can stay in the sun without burning while using a sunscreen versus how long you can stay without burning while wearing NO sun protection.
Say that your skin normally begins to turn red after 10 minutes of unprotected sun exposure. If you use a product rated SPF 2, your skin would not begin to burn for twice that amount of time: 20 minutes. If you use a sun product rated SPF 25, you could stay in the sun for 250 minutes, a little over 4 hours (25 x 10 minutes) before your skin would start to turn red. Remember, though, that SPF numbers are not absolute. It all depends on how fast or how slowly YOU start to burn. If your unprotected skin burns in 5 minutes, an SPF 25 product might let you stay in the sun a little over 2 hours without burning (25 x 5 minutes = 125 minutes).
Here's something else to keep in mind: The SPF measures only the level of protection from UVB rays, the ultraviolet rays that cause sunburn, and does not measure levels for the far more dangerous UVA rays, which penetrate the skin more deeply and can cause damage at the cellular level. When choosing a sunscreen, it's important to look for one that offers broad-spectrum protection from both UVA and UVB rays.