What does this tool do?
It is a simple free HTML validator which is based on specifications provided by w3.org. This tool provides guidance on markup errors and best practices on Web documents in HTML, XHTML, SMIL, MathM in a clean and easy to read format.
Why should I correct errors in my code?
Granted, most popular modern browsers such as Google Chrome will be able to render a web page with erroneous code perfectly well, however, some browsers may not if the code falls outside of W3C specifications.
One of the most common problems with poorly written HTML is miss-matched nesting, which in any case is simply unacceptable. Aside from that keeping your code inline with W3C standards is just a good thing to do and will make you a more proficient front-end developer.