From Wsms

mandb replaces makwhatis on recent Linuxmint/Ubuntu/Debian distributions. Its purpose is to rebuild the man database uses by the apropos or man -k commnand.

You usually don't need to run mandb because it is run automatically by a cron command. You may want to run it if you install some new commands and want to look at their man pages right away.

common usage

Install emacs:

root@mckinley:~# mandb
...

see also

man man

man apropos

man mandb