Now that it is Summer, you may have noticed your skin becoming a tad darker. If you are normally fair-skinned, then the sun has likely tanned your most sun-exposed skin. So, you might ask, what is the science behind this tan?
The sun has increased its melanin production in order to protect itself. Melanocytes in the skin (also found in the eye as discussed here) make melanin, which gives skin its color. When sun hits the melanin in our skin, the melanin soaks up the harmful ultraviolet (UV) rays. The more time we spend in the sun (typically during Summer months), the more melanin is made by our melanocytes in order to protect us from all of the extra UV rays. Eventually the tan goes away (those who are fair-skinned become "pasty" again) because our skin cells are rapidly dividing/turning-over and eventually the melanin-rich cells die-off. And without the blazing sun around, there is no need for excess melanin to be made; therefore our melanocytes continue during the rest of the year to simply produce 'regular' amounts of the protective melanin.
In addition, if you are a freckle-bearing individual, you may notice that the sun has brought out some more of them too. Same mechanism applies here. Freckles are simply sites of concentrated melanin. More sun, means more melanin, means more freckles, means more sun protection.