SPF, also known as Sun Protection Factor, measures how well a sunscreen can protect against skin damage from UV radiation. Sunscreens are made up of various organic and inorganic compounds. These compounds are able to absorb UV radiation and release its energy in the form of heat. 

Most sunscreens indicate a SPF value. This value represents the ratio of the amount of UV radiation required to cause erythema (redness of the skin) on sunscreen protected skin to the amount of UV radiation required to cause erythema without the sunscreen. 

For example, if it normally takes you ten minutes under the sun to develop skin redness, wearing a sunscreen with SPF 15 will allow you to be under the sun for 10 x 15= 150 minutes before developing skin redness. Therefore, sunscreens with higher SPF values tend to be more efficient at protecting the skin from UV damage than sunscreens with low SPF values.   

Reference:
El-Boury, S., Couteau, C., Boulande, L., Paparis, E., & Coiffard, L.J.M (2007). Effect of the combination of organic and inorganic filters on the Sun Protection Factor (SPF) determined by in vivo method. International Journal of Pharmaceutics, 340: 1-5. 

Image source: 
http://www.skincancer.org/prevention/sun-protection/sunscreen

Although I've often tried the "slow and steady" approach to studying for an exam, I, like many others, have often found myself struggling with this strategy and preferring the "last minute" approach. There may be some logic to why this strategy might work. 

Stress associated with exams is usually highest right before an exam. Studies have shown that (up to a certain point) there is a positive association between level of stress and efficiency in learning and memory. This is because stress induces the release of certain hormones in the central nervous system that help improve memory. 

However, stress only seems to be able to improve memory under the following conditions:

1) When it is present at the time of learning and/or memorization.
2) When the stressful situation is directly related to what is to be learned.

On the contrary, high levels of stress before or after the learning event may actually decrease learning and memorization.    

Therefore, the most efficient studying may be accomplished right before an exam, which explains why some people may prefer cramming to the "slow and steady" approach.

Reference: 
Joels, M., Pu, Z, Wiegert, O., Oitzi, M.S., & Krugers, H.J. (2006). Learning under stress: how does it work? TRENDS in Cognitive Science, 10(4): 152-158.