Documented in Volume 1 of the UNIX Programmers Manual.
% apropos font
will scan manuals sections and list title lines that contain
the string 'font', for example:
fed (1) - font editor
vfont (5) - font formats for the Benson-Varian or Versatec
vfontinfo (1) - inspect and print out information about UNIX fonts
vwidth (1) - make troff width table for a font
% apropos password
will scan and list any title lines which contain the
string 'password'.
% apropos editor
will scan and list lines for the string 'editor'.