What is "valid" web code?

  
Seen these icons? This is what they mean:

In the context of websites, valid code means that the (x)html is error free in syntax. (x)html is a computer language. All languages have their own grammar, vocabulary and syntax. Validation does not refer to the content, but rather that the mark-up of the page meets the basic requirements for industry standards.

What does Validation do?
Some claim that validation helps to increase traffic and improve rankings with search engines. Not necessarily so. That's like saying that a car with a smooth running motor will attract riders and turn heads. Mechanics would certainly take notice, and if you needed a dependable ride, such a car would likely get you where you're going. But it doesn't stand to reason that you'll get there in style. Thus, validation is the declaration that it is indeed, a well-oiled machine. What is DONE with it is another matter.

Why is Validation important?
Valid web code helps browsers know how to render the page. If the web code contains errors in syntax, then browsers are left to guess what the programmer probably meant by the inclusion or ommission of an element. Therefore, an error in syntax can result in an error in rendering the page. While browsers are smart about some things, I personally wouldn't leave the rendering of a layout open for interpretation by a piece of software.

How do you check Validation?
The World Wide Web Consortium has a free validation service. Simply enter a URI and it checks the syntax for errors, returning a "red light" or "green light" indication of whether the page passed the test.

So, when you see these icons, it means that the web developer took the extra time to ensure that what's under the hood is running optimally. In web design, just like with fancy cars, no one likes a poser.