Setting a good root password is the most basic requirement for having a secure system. See passwd(1) for some hints on how to create good passwords. You can also use an automatic password generation program to do this for you (see 第 4.11.20 節「Generating user passwords」).

Plenty of information on choosing good passwords can be found on the Internet; two that provide a decent summary and rationale are Eric Wolfram's http://wolfram.org/writing/howto/password.html and Walter Belgers' http://www.belgers.com/write/pwseceng.txt