Sampling is choosing which points you want to have represent a given image. Given an analog image, sampling represents a mapping of the image from a continuum of points in space (and possibly time, if it is a moving image) to a discrete set. Given a digital image, sampling represents a mapping from one discrete set of points to another (smaller) set.
Here's an example of an original image [Weston] , and the same image with every third pixel sampled. The intermediate pixels are filled in with the sampled values.
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