In simple terms SD is a ratio of a bullets diameter vs it's weight. To picture it's importance imagine a dropping a soda can on your bare foot; no biggie ,right.
Now picture dropping a 3/16" diameter steel rod that weighs the same as the pop can onto your bare foot end first. OUCH
that's SD at work
This same principal is also in effect with relation to a bullets slicing through the air. SD is a large variable in determining a bullets BALLISTIC COEFFICIENT
another way to expain it would be, would you rather have Roger Clemens throw a wiffle ball at you ,filled with sponge material, or the same size ball, made of lead?
Rangerruck to be perfectly accurate, the basic notion of SD is that if you had a bullet made out of a heavy material, it would have the same SD as a bullet made from a material 10x as light, if the light bullet was 10x as long. That's the sectional aspect, you're sectioning it on a horizontal plane. If the bullet was traveling sideways the SD would be much much less, because the section is thinner.
Just to be clear so grafsk8er doesn't think SD just means that light materials are low SD and dense ones are high SD, because it's not always true. There are very high SD aluminum and copper bullets. The basic notion of sectional density is not the material used, but the mass of the material over the length of the section.
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