First off, it’s “Canon” with just one N (well, two total), not “Cannon,” which is a large gun.
The very short answer is, some priests are members of particular groups of clergy called chapters, which are usually affiliated with a cathedral. Members of a chapter are traditionally called “Canons,” but this is not a separate level of the priesthood or something. For a longer answer, take a look through this Wikipedia article and this Catholic Encyclopedia article and see if they resolve your question.
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