Find out what a command does

At the command line, run:

whatis

This will give you a one-line summary of a command.

If whatis doesn't work, the whatis database may not be built yet. To build the whatis database, enter this:

/usr/sbin/makewhatis

I can't remember the name of the command I need

The apropos command searches a database of commands, using keywords:

apropos

Note that your keyword might not work. So try different keywords until it does!

If apropos does not work, you probably need to rebuild the whatis database with this command:

/usr/sbin/makewhatis