whatis looks up a given command and displays the header line from the manual section. You can then run the man(1) command to get more information. If the line starts `name(section) ...' you can do `man -s section name' to get the documentation for it. Try `whatis ed' and then you should do `man -s 1 ed' to get the manual page for ed(1).
whatis is actually just the -f option to the man(1) command.
whatis uses the /usr/share/man/windex database. This database is created by catman(1M). If this database does not exist, whatis will fail.
/usr/share/man/windex Table of contents and keyword database
See attributes(5) for descriptions of the following attributes:
|ATTRIBUTE TYPE||ATTRIBUTE VALUE|
apropos(1), man(1), catman(1M), attributes(5)