Interface administrators can edit CSS/JS pages that they do not own (that is, any page ending with
.js that is either in the
MediaWiki: namespace or is another user's user subpage).
In particular, they have the ability to edit sitewide CSS/JS pages (pages such as MediaWiki:Common.js or MediaWiki:Vector.css, or the gadget pages listed on Special:Gadgets).
These pages are executed by the browser of wiki editors and readers as code, which can be used to change how content is styled, change the behavior of pages, or even create highly complex tools.
In addition, they can also edit all other pages in the MediaWiki namespace.
Editing CSS/JS that gets executed in other users' browsers is very powerful and potentially dangerous in the hands of a malicious user. Interface administrators should be users who need it, who are trusted by the community, and who follow common basic password and computer security practices (use strong passwords, do not reuse passwords, use two-factor authentication if possible, do not install software of questionable origin on your machine, use antivirus software if that's a standard thing in your environment).