Second curtain flash (also known as rear curtain flash) is a camera setting found on top-of-the-line 35mm (and digital) SLR and quite a few medium format photo cameras. The setting ensures that flash photography of moving objects while using long shutter times behaves the way we would expect the result to be.
On a regular flashgun exposure, the flash fires when the shutter opens. This is perfectly acceptable for 99% of all flash photography, but some times you would want the flash to fire just as the shutter closes;
Imagine that you move your arm very fast downwards, in a Karate chop where you try to break a log of wood. If you took a picture of that, using a long shutter time and a flash. What would you expect the result to be? If you use a regular (first curtain) flash system, you will in fact see the hand clearly on top of the movenement, while the whole wood-chopping action would be a blur of motion.
If you use second curtain, you would freeze the hand's motion just where the motion stops - in other words, you get the blurred motion of the arm, ending in a "frozen" image of the arm on its way down.
When you see the "speed lines" behind cartoon characters, the cartoonist is in fact mimicking the second shutter flash effect.