SPA is an acronym for "Single Page App". What that means is that when you go to a web page it loads the website once, and then from there on out only updates are rendered. Think about Facebook. When you go to facebook you scroll on the newsfeed, view posts, and message people in your browser and the screen never flickers and you never need to hard refresh. The page just incrementally updates the experience feels smooth.
These frameworks have taken over. The site you are reading this on was made using React, and almost any site with any interactive complexity is a SPA written using one of these technologies.
Did you notice when you click around on this site it loads super fast? That is because it is using React + React-Router and everything is already loaded, it is just re-rendering locally when you request new pages.