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A description of the shape and size of the tool used to create a pad or track. The term comes from the days of vector photo plotters, where film was exposed by shining light through apertures (shaped holes) arrayed around the edge of a disk (or "aperture wheel"). Each aperture corresponded to a different D code in the Gerber data. Today, photoplotters use lasers to expose the film but the term "aperture" persists.
D-codes are instructions to the photoplotter that naturally include the letter "D." The first three D-codes control the movement of the x-y table.
Anyways,check this link below and you will understand better,it also covers all about gerber files..