validation is the process of checking if something satisfies a certain criterion [Wikipedia].
validation is always nice to receive ... it's the same for web sites. Governing bodies have set standards for web design in the areas of content delivery, formatting, language encoding and accessibility, among many others.
some of the most important standards are those relating to html (the framework that describes how your web site looks in a browser), CSS (Cascading Style Sheets, which help lay out the content and make it pretty), and accessibility (making the content easy for people with disabilities to navigate your site). Luckily, there are many online validation tools that web developers can use to ensure that their web sites validate against these criteria:
when a web site can successfully validate against these standards, the owners can be assured that they have a widely viewable site that will perform well no matter which browser or operating system is being used (within reason, of course).