What are front and rear curtain sync ?

When you reduce the shutter speed setting on your camera it makes a difference whether you have the flash firing at the beginning (front) or the end (rear) of the exposure. Sports photographers prefer to use rear as it means the ambient light will reach the sensor first and then the flash is pinged over the top. If it’s captured the other way around the ambient blur looks like it’s going in the wrong direction. With front and rear curtain sync, the longer the shutter speed you use, the more you’ll notice this ambient blur effect.