Hello,

as per the title and description of the addon, regular expressions are used to evaluate the urls, and all the contents of the input fields are treated as such. Information on regular expressions is widely available, here are a couple resources:

Mozilla Developer Network: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Cheatsheet
An interactive tool: https://regexr.com/

As for your question, a regular expression matches a string (in this case, a url) if it matches any portion of the string. Therefore, entering "mozilla" in the url field would match all Mozilla domains, as well as any other url containing that word, such as, for example, a news article about mozilla with a portion of the title in the url. The expressions need only be long and comprehensive enough to disambiguate between intended and unintended matches.
Clicking on the eye icon of a given rule performs a dry run for that rule and displays the would-be changes to your browsing history in a new tab.

I hope this helps.