Pre-rendering is the act of processing Javascript to build a page ahead of time, creating an HTML file, and then providing that to whoever requests it. It’s a premade page that means nobody—client or server—has to process any Javascript code as part of the page request and load process.
Prerendering is really useful if, for example, you have a data source that doesn’t change very often, but the process for checking the most up-to-date price takes a long time to run.