What is an uncategorized URL and is this something to worry about for a standard niche website?
An uncategorized URL is an URL without any category, logical right?
For example, in a blog (about webmastering), you can categorize articles (thus URL) into categories (HTML, CSS, SEO, etc.) and get URLs like this:
However, you can have some pages you don't want to classify because it's not relevant; for example the contact page. In this case, the contact page is uncategorized and you have this URL:
www.example.com/contact/ (no special category). It's totally normal and in general, uncategorized pages are not harmful at all for SEO. They are very strong pages for a website because in general, they get a backlink from all pages of the website.