What one thing ruins more pictures than anything else? The blur that results from an out-of-focus image. Holding the camera correctly can help prevent that blur. Wrapping your left hand over the top of the lens makes it less stable and more likely that you’ll move the camera while shooting.
A very fast shutter speed would be 1/1000 second, and that speed would freeze most action in its tracks. A slow shutter speed like 1/30 second would blur a person casually strolling by. Most cameras offer a wide range of shutter speeds, from as long as 30 seconds to as short as 1/8000 second.