Hello!
Having a high Quality Score means what Google thinks about your advert, if is relevant and useful to someone looking at them.
You can find out your Quality Score for any of your keywords.
Let's say that you are selling only Semi-Dedicated Hosting. If a user searches on Google "Semi-Dedicated Hosting" keyphrase it should see you, because you are selling "Semi-Dedicated Hosting" and your keyword is "Semi-Dedicated Hosting". The user searches and finds exactly what he's looking for: "Semi-Dedicated Hosting" => High Quality Score. If on the same page, a competitor appears with, for example, "Hosting" it will have a Low Quality Score, because it doesn't represents exactly what visitor's looking for.
It depends on:
CTR
The relevance of each keyword to ad group.
The quality/relevance of landing page
The relevance of your ad text
Your historical AdWords performance
Hope this helps!