Shooting in darker conditions means a photographer has to do one of two things, use a slower shutter speed or reach for a flash. Slower shutter speeds are fine for stationary subjects but will cause moving subjects to blur. Conversely, shooting in the dark with a flash will produce dark backgrounds which may not be the desired look. Slow-sync flash means your flash fires and your camera uses a slower shutter speed, meaning you have the best of both worlds.
As your flash goes off it lights up your subject, while the slower shutter speed used provides better exposure for your background. Mastering this technique does take some time, and you will need to make some other decisions and changes, such as using a tripod or panning with a moving subject and adjusting the shutter speed for effect. All things considered, this is a fun and rewarding technique that can vastly improve your photographs with your pop-up flash.