The Secunia PSI, or Personal Software Inspector, is a program that checks all the installed software you have on your computer, and tells you if you have the latest edition with all the updates installed. Most hacker exploits different code flaws or vulnerabilities in software to gain access to the computer, and most software companies make updates or new versions of their software when any such code flaws are discovered. Because of this, it is important that you have the latest version installed.
By keeping all your software up to date, you are greatly reducing the chance of an attacker getting access to your computer. If you also have a firewall and updated virus protection, you should be well equiped against most kinds of attacks to your system.