The number 1 rule for any sys admin, is to keep systems and servers up-to-date with the latest security patches. (number 2 rule is to create regular back-ups).
FreeBSD 6.1 suffers from classical check/use race condition on SMP. The bug was fixed in 6.1-STABLE, just before release of 6.2-RELEASE, but was not recognised as security vulnerability.
This code exploits this vulnerability to run root shell.
To find out more about FreeBSD security, refer to the FreeBSD Security Information page.