What does the external flash of SLR camera mean by "synchronization"?

The so-called "synchronization" of camera flash means that the flash flashes at the same time when the shutter is released. Because the shutter of the SLR camera is designed in the form of a curtain, which is divided into "front curtain" and "Hou Lian", when the front curtain is opened and moves from top to bottom, the Hou Lian moves with it, and a small slit also allows the light to "sweep" the whole film or photosensitive element from top to bottom, thus completing the whole exposure process. If the shutter speed is too fast, the flash will go out before the cracks in the middle of the front and back curtains completely "sweep" the film or photosensitive element, leaving a horizontal strip shadow on the photo (that is, the part that has not been irradiated by the flash). Most of the front and rear curtains of early SLR cameras were designed to move horizontally, and vertical black bars were left in the photos. In order to solve this problem, the flash shutter speed of SLR cameras is usually designed as1/60s,1/90s or1/25s, so as to ensure that the film or photosensitive element is completely exposed during the whole shutter movement. When taking pictures with the automatic function of SLR camera, the camera will automatically fix the shutter speed at the synchronous shutter speed; If you use the manual function to take pictures, if the shutter speed is set higher than the flash synchronous shutter, the flash will not be synchronized.