Burst photos happen when you hold down your shutter button for a period of time. Your iPhone will automatically take a rapid succession of pictures, that you can get at later. So you can capture a still shot from any type of motion that’s happening. Burst is a great way to capture both action and group shots. It uses something such as, ‘Smile and Blink Detection’ that works in conjunction with your face detection feature.
Burst Mode will take photos at up to 10 frames per second on the 5S, so you don't miss the best shot. This comes in especially useful when taking a photo of a child, or an action shot and timing isn't easy. https://www.cnet.com/how-to/how-to-use-burst-mode-on-the-iphone-5s/