What exactly exposure means..

When we talk about exposure,we refer to how long the shutter of the camera is let open.The speed with which shutter behaves(i.e opens or closes),determines for how long the digital sensor in a camera is exposed.
This is generally determined by the shutter speed in the camera(which can be accessed in camera's with manual settings)
shutter speeds vary broadly b/w :- 1/4000 of a second to 30 seconds.
1/4000 means that the shutter speed is very fast and camera's digital sensor is exposed for only 1/4000 seconds.
30 indicates very slow shutter speed,in which the digital sensor is exposed to light for 30 seconds.
We normally give 30 sec.exposures,in case light conditions are dim and we require more detailing.(as in case the pictures down the line)
For shooting fast moving objects,we prefer to go for smaller exposures(1/2000 , 1/4000 , etc..)