This is a trait you can add to your theme's PageTheme class if it is built with the Concrete bedrock. That means it includes the bedrock SCSS and JS files. If your theme's JS and CSS files include these starter assets, you'll automatically need to require jQuery and Bootstrap4, and you'll automatically support the bootstrap4 grid system.
When activating a theme, any file within the theme is loaded into the system as a Page Theme File. At that point the file can then be used to create a new page type.
Class RouteCollection. Holds specific special route/theme combinations. These are functions that used to live in the Router class but it makes no sense for them to take up space there.