Shutter speed refers to the time the camera’s shutter is open. Usually denoted in seconds or fractions of a second, such as 1s, 0.5s, or 1/320.
When the shutter is open, the sensor (or frame in film photography) is exposing. For example, a photo shot at 1/1000 means that light is hitting the sensor for one thousandth of a second.
Shutter speed affects the sharpness of an image. The longer the shutter is open, there is more time and opportunity for the camera to move, making the image blurry. This is caused by photons from the same origin moving across the sensor and exposing different pixels. The same is also true for a moving subject, even if the camera is not moving.
Shorter shutter speeds (larger fractions of a second) makes capturing moving subjects (or using a moving camera) easier, but also requires a larger aperture, higher ISO, or both in order to capture a photo with the same level of exposure.
Short shutter speeds can produce images that have the effect of freezing a fast-moving subject, or keeping the subject in focus while the environment becomes blurred with motion. Alternatively, “long exposure” photos (photos with longer shutter speeds) can be used to create images with intentional blur, or light painting photos.