What is shutter speed? Does it have anything to do with the flash?

The shutter opens to let light through to the film or electronic sensor, and closes again. If you alter the length of time the shutter stays open, you can drastically change the kind of image you'll end up with.

At slow shutter speeds -- when the shutter stays open for a longer time -- objects in motion will appear blurry in the photograph (and everything will get blurry if the camera itself isn't held still). That's because the same object is recorded in multiple places on the film or sensor as the object moves across the frame during the time that the shutter is open. Second, the longer the shutter is open, the more light reaches the film/sensor, resulting in a brighter overall image.

The Shutter speed is very important, and the speed have a directproportion with the amount of light , increasing the shutter speed , needs a lot of light , and at the same time it helps you to shoot fast movements with out jerky picture or blurness , so the foottage will be net and no jittering