The Sun Protection factor (SPF) gives an indication of the amount of protection sun cream offers against UVB radiation. Many other factors will impact how long you can stay in the sun without burning, such as your skin type, eye and hair colour, where you are in the world, altitude and lots of other things.
After applying SPF 30, you are able to stay in the sun 30 times longer than you could without wearing sun protection. Be sure to wear the directed amount of sun protection (always more than you think, this should be around 30ml per application for an adult). Higher factors should not be used to increase your time in the sun.
A common misconception is that SPF 30 is twice as strong as SPF 15 and so on; this is not true. SPF 15 blocks 93% of UVB radiation whilst SPF 30 blocks 97% and SPF 50 blocks 98%.