Searches through headers of manpages for a string. It is equivalent to man -k.
Apropos is useful when you have some idea of what you want to do, but don't know the name of the tool required. For example: "apropos rename" gives lots of results, including this useful one:
mv (1) - move (rename) files
Apropos is quite closely related to whatis and uses the same database, which is created by the makewhatis command. Neither command will function in a completely new system until makewhatis has been run for the first time.
Because apropos does a simple string search on the headers, it is liable to retrieve quite a large list of man pages, which you might want to pipe through a pager such as more or less. This article is a stub and needs to be finished. Plunge forward and help it grow!