Shutter Speed

     Because light levels change, digital cameras must vary the amount of light reaching the sensor. A shutter is a mechanical device that opens and closes quickly, altering the length of time that the sensor is exposed to the light.

This is the amount of time that the camera’s shutter remains open. The longer the shutter is open, the more light can enter and fall on the sensor. If the shutter is open too long, the image will be overexposed (there will be too much light entering and hitting the sensor) or in other words burned out. Overexposure means the highlights are washed out, there is no detail in them, only white pixels. But if the shutter isn’t open long enough, the exact opposite happens: there won’t be enough light for the sensor to register, so the image will be too dark (underexposed).
 

Starting with the Basics - Exposure triangle

Exposure determines how light or dark an image will appear after it's been captured by your camera. It is nothing more than the amount of photons available for capture by the sensor. If there are too few photons, the image doesn’t register at all (underexposed). Too many photons, and the photo will be overexposed.

Exposure is determined by 3 camera settings: Shutter Speed, Aperture and ISO; this is called the Exposure Triangle.