The SPF refers to the ability of a sunscreen to block UVB rays that can cause sunburns, as well as UVA rays, that can cause deeper skin damage. The higher the SPF number, the higher the protection. But none provides 100% protection. The number shows the length of time your skin will be protected from burning, depending on your skin type. Check for the first sign of reddening in the sun without any protection (for eg., 3mins). Multiply that number by the SPF, that is, 50 * 3 (if the SPF is 50) = 150 minutes of sun protection.