"Common knowledge" refers to information or facts that are found in many places, or to things that the general public could be expected to know. A good rule to follow is, if you had to look something up to verify it (even if it was simply in an online encyclopedia), you should include a citation for that information (i.e., it is not something that the average person would know off-hand; it is not common knowledge). Facts that could be considered common knowledge do not need to be documented with a citation in your paper. It would not be necessary, for example, to cite a source when saying that pirates robbed ships, since many (if not most) people are aware of this. However, ideas, facts or statistics that are not commonly known, analysis or interpretation of ideas or facts, and ideas or facts that pertain directly to your argument should be documented with a citation.
As always, when in doubt, cite the source!