What is a "nib", as in "mainmenu.nib"?

A “nib” is an “Interface Builder” file. The AppleScript Studio projects you create in “Xcode” may contain “nib” files, which contain all of your interface elements. A “mainmenu.nib” file is included in new “AppleScript Studio” projects.

From “Xcode”, you can open a “nib” in “Interface Builder” by double clicking on it.