For more information visit: http://www.ukfast.co.uk/blog/2014/08/28/what-is-vulnerability-scanning/
What is a vulnerability scan?
The bad guys are constantly scanning for vulnerabilities in systems out there, to find out who is already open to attack. The idea of a vulnerability scan is that you find the weaknesses and fix them before they get to you. Whether it’s in your network, infrastructure, or web application, a vulnerability is normally a flaw in your code, which attackers will use to break into your system before proceeding to totally rinse you.
It’s often used as part of a two-step process along with penetration testing, during which ethical hackers try to break in using the flaws found in the vulnerability scan to help you figure out how to fix them.
Why do you need one?
Aside from letting you know where you need to reinforce your system, thereby protecting you and your customers, it can help with becoming PCI-compliant.
You can watch this video also at the source.