What is whatis?
whatis command displays a single line information about a command.
Display single line information about a command
Use whatis command to display single line information about all the keyword matches of passwd.
$ whatis ifconfig ifconfig (8) - configure a network interface
When there are more than one man page available for a particular command, it will displays single line information from all those man pages as shown below.
$ whatis passwd passwd (1) - update a user's authentication tokens(s) passwd (5) - password file passwd [sslpasswd] (1ssl) - compute password hashes