What is delayed flash in photography?

When taking pictures in dim light, if you use a flash and a high shutter speed, it is easy to cause the foreground subject to be too bright or even dazzling, while the background is still gloomy and it is impossible to distinguish the details. "Slow flash synchronization" will delay the release speed of the shutter of a digital camera. The flash will illuminate the foreground, and with the slow shutter (such as 1/2 seconds or several seconds), both the foreground and the background will be fully exposed.