"Citators are a catalogued list of cases, statutes, and other legal sources showing the subsequent history and current precedential value of those sources.
Citators allow researchers to verify the authority of a precedent and to find additional sources relating to a given subject.
Citators were originally printed on gummed paper and pasted next to the report of a cited case. Today, citators are published in volumes and are also available online; the two most popular are Shepard's and KeyCite."
Source: Black's Law Dictionary. 8th. St. Paul, MN: West, 2004
In simple terms, citators help to answer the question, "Is this still good law?"