It means your site is not coded to standards but actually to cater to IE's quirks. By providing a doctype IE jumps out of the quirks into as-close-to-standards-as-we-can-get mode and your page breaks. Read the article given to you, put a doctype on your page (first thing) and start redesigning. If you think that is a stupid thing to do, be aware that your page was probably looking broken in every other browser but IE already.