Buefy (pronounced /ˈbyo͞oˌfī/, like beau-fy) is a user interface component library made on top of two fresh but very well known frameworks, being them Vue.js and Bulma. Buefy has two core principles: 1. keep stuff simple and 2. be lightweight.
This documentation assumes you know the basic of Vue.js, if you're not confortable yet with it, it's probably not the best idea to learn from here as your first step — learn the basics then come back. Vue.js website is the best source for you to start with.
Now that you know what Buefy is, get on next step and install it!