The ctags program generate an index (or "tag") file for a variety of
language objects found in files. This tag file allows these items to
be quickly and easily located by a text editor or other utility. A
"tag" signifies a language object for which an index entry is
available (or, alternatively, the index entry created for that object).

Alternatively, ctags can generate a cross reference file which lists, in
human readable form, information about the various source objects found in
a set of language files.