A barn door of time
One of the fascinating things about flash photography is that your flash, besides being able to put out an impressive amount of light, is also able to do so in a very short interval of time. In fact, it is so fast that relative to a shutter speed of 1/20th second, the flash sees this as a barn door of time!
Decisions, decisions
Provided that the flash is fast and the shutter is slow (even when fast!), this gives an interval – a string of options for when to fire the flash when the shutter is open. Most cameras will default to firing the flash at the beginning of the shutter being open, but there typically is also an option available in your camera settings to fire the flash towards the end of the shutter being open. So the “string” of options is on most cameras reduced to two options: early or late. The late one is often called something like rear curtain sync.
Subject move
If you take a picture in a low light situation where you need to keep the shutter open for say 1/10th of a second, you will see that anything bright and moving will leave a trail of light. So the headlights on a car on a highway at night will leave a trail of lights as the car moves through the frame.
Now on top of this ambient exposure comes the flash. And it can be added early or late in the exposure. Again, remember that the flash is blazing fast and only uses a tiny, tiny fraction of the 1/10th in the example. Provided the flash is strong enough, it will add an additional exposure of the car, where the car stands out much stronger relative to the exposure done by the ambient light. In fact, the car may not be visible at all in the ambient light exposure – only the headlights.
So here is the thing: do you want the car to be exposed as the headlights start to be captured or towards the end of the headlights being captured (= rear curtain synchronization). That is basically what the rear curtain sync option governs. Most cameras will default to early sync, and have rear sync as an option you need to actively select.
If this sounds all mysterious and convoluted, my best advice is to give it a try. Shoot something bright moving at night and see the effect of turning rear sync on and off. It is the best teacher. Far better than any blogger! Enjoy!