Stay up-to-date with the latest security fixes.
There is an old saying that the only safe computer is one that's disconnected from the network, turned off, and locked in an underground bunker—and even then you can't be sure! Keeping your computer up-to-date with the latest security patches is essential if you want to keep yourself safe from the latest threats and exploits.
When each version of Ubuntu is released, all packages within it are considered "frozen." No new versions of software contained in that release are added to it, so when you install Ubuntu Dapper Drake, the versions of all the software available within that release will remain the same indefinitely. New versions of individual packages are not added because that would make the release a moving target rather than a predictable environment—and might even introduce new bugs and security vulnerabilities.
Of course, software itself doesn't stand still; new versions are always coming out, and sometimes an existing vulnerability is found and fixed by a new release. That means older versions of the software may still be vulnerable, but Ubuntu policy dictates that new versions of software are not shoe-horned into an already released distribution.
This impasse is resolved by back-porting security fixes to the version of the software that was included with the distribution at the time of release and then releasing a "security update" package for just that particular piece of ...