Website Quality at a Glance is a tool which gives you a quick indication of the technical quality of the tested webpage. No more, no less.
To do this quickly only a few aspects of the webpage are considered when testing the webpage.
Note that this technique is also used to find out if we should tip a link in the directory.
If you want to know in detail what to do to improve the quality of a webpage probably one of the following tools might help.
Tip: Use a bookmarklet to get the quality score even quicker.