Not much at all, all .vue files in the components directory gets registered globally, so if you create a Navbar.vue file, you can use <Navbar /> anywhere in your theme. Easiest templating system I've ever used. If you learn a bit of how passing props work, you're golden!
Do you know if registered globally = pre-loaded or are they only loaded as encountered / needed? Just wondering if you have to be careful how many components you put in that directory. Not for purpose of templates of course, but for the unique Vue components that you can put in page content.
Some info on vue single file components: https://vuejs.org/v2/guide/single-file-components.html
Props documentation: https://vuejs.org/v2/guide/components-props.html