This is a tracking category for CS1 citations that are "empty".
Empty citation
A Citation Style 1 or Citation Style 2 template marked as "Empty" contains no recognizable parameter identifiers. For example, this citation is marked as "empty" even though it contains usable information:
{{cite web |http://www.foobar.com |The Foobar News}}The citation is "empty" because it does not contain the necessary parameter identifiers (in this case |url= and |title=) that tell {{cite web}} how to use the information that the citation contains.
A citation is also marked as "empty" when it contains only parameters that it doesn't recognize:
{{cite book |titolo=The Bazquux Uprising |anno=2015}}This citation is "empty" because the Italian-language parameters |titolo= and |anno= are not recognized as valid parameter names on the English Wikipedia.
"Empty citation" may also mean that an editor meant to use another template entirely. Perhaps the editor used {{citation}} when the intent was to use {{citation needed}} or {{quote}}.
To resolve this error, add appropriate parameter identifiers, translate foreign language parameter names to their English equivalents, or replace the CS1/CS2 template with a more appropriate template.
Pages with this error are automatically placed in Category:CS1 errors: empty citation.