A camera flash is either and internal or external light that illuminates the scene for better exposure. Flashes come in a myriad of styles, configurations, and capabilities.
Many digital cameras come with a built-in flash, and most cameras come with the capability of working with external flash units. Cameras with the ability to fire external flash units are the standard in studio photography, and in situations where there needs to be either off-camera lighting, or lighting from multiple sources.
Flash sync is the synchronization between the shutter exposing the sensor of the camera, and the flash being fired. Most flashes operate at 1/60th of a second, which means that if the shutter is open for less than that time, the sensor will not fully expose, resulting in black bars at the top and/or bottom of your photo.
Flash Sync Speed
Modern DSLR cameras offer “flash sync” shutter speeds that enable faster shutter speeds than would otherwise be possible to use with a flash. Flash Sync controls your camera operate differently than usual: while in normal operation, the sensor curtains (two frames inside the camera body that cover the sensor) open to expose the sensor fully to the image. With flash sync, the sensor curtains closely follow each other so that only part of the sensor is exposed. This operation happens faster than a flash. This enables using a much faster shutter speed while still being able to use a flash.
Cameras that offer flash sync have traditionally synchronized via a wire running from the camera to the flash unit. Modern technologies have enabled external flash units to be synced wirelessly. Wireless flash devices use various methods of syncing: radio transmission (either proprietary or a widespread technology like bluetooth), sensing other flashes fired, or infrared.