WebSockets are a new technology that enables two-way communication between the user and the server where the app is hosted. Before this technology, only the browser could initiate a request and, thus, a connection. If some update on the page was expected, the browser had to continuously poll the server. With WebSockets, this is no longer necessary; after the connection is established, the server can send updates only when there is a need.
You don't need any preparation for this recipe, just the basics of Vue. If you don't know what WebSockets are, you don't really need to, just think about them as a channel of continuous two-way communication between a server and browser.