Consider your website's code like handwriting. If someone had to had to scan over a lot of writing in a short time, there is a chance that they would not be able to absorb as much information from a paper will sloppy handwriting vs. a paper with neat and organized handwriting.
The same can be said for the crawlers that come visit your site. They have to go through possibly billions of pages, and chances are, they will not spend too much time on a page where they cannot decipher the content due to bad, unorganized coding. A page with clean, organized code is likely to be crawled from the header to the footer.
One place to check your site's code to make sure it is valid is using the free markup validation tool by W3C, pictured above. This site will give you warnings, errors, and suggested ways to correct the incorrect code in question.
Even if you are not able to immediately clean up the code throughout your site right away, at least take a look at the errors and learn proper coding for new pages.
