stop action
Explanation from the Nikon website:
One thing that makes photography unique is its ability to freeze a moment in time. As humans, we see what's in front of us as a continuing chain of events. The camera, however, is able to stop time from moving forward, giving us the opportunity to study that fraction of a second. And it's being able to control those fractions of a second that makes this possible.
There are two elements in every camera that affect how much light enters the camera and finds its way through the lens to the camera's sensor. One is the aperture and the other is the shutter.
The aperture is the opening that can be adjusted to allow more or less light to enter the camera. The varying sizes of the opening are called f/stops and are referred ao in numerical terms: f/4, f/5.6, /8, f/11 and so forth.
The shutter is the mechanism that determines how fast the aperture will open and close. The slower the shutter speed the more light that enters the camera. Shutter speeds are measured in seconds. Most cameras have shutter speeds that range from several seconds-some as long as 30 seconds-to fractions of a second-up 1/1000 of a second and above.
The camera's shutter speed determines how the action in a photo is recorded. A fast shutter speed can freeze action, even stopping a hummingbird's wings. A slow shutter speed can let action blur or even make a moving subject seem to disappear. First, let's look at how a high shutter speed can stop action.
When you want to take a picture that stops the action of your subject, you have to first consider the speed of the action. The speed of the blades of a helicopter in flight is much greater than that of a kid biking down the street. Trying to freeze those helicopter blades might require a shutter speed above 1/1000 second; stopping the movement of the bike might be possible at 1/250. Thus, the shutter speed you choose has to be relative to the action you're trying to stop. A good rule of thumb would be that 1/500 second is a good starting point for stopping motion that's fairly fast.