A SPA is a Single Page Application. It is exactly what the name implies.
As modern web design has come a long way, so also has JavaScript; it is now possible (or much more feasible) to package the functionality of an entire web application spanning many pages into one page, and have a reactive JavaScript library update the page in realtime.
Who cares?
Well, you care, because if the entire website loads at once, this means you don't have to wait for page reloads.
Notice in the Crappy Server Pages every time you click on a button on the pagination component, it reloads the page. But in the SPA version, changing pages does not cause a page reload. That is the standard practice for updating a server page (or at least, most server page frameworks are designed in such a way that the simplest way to update a page is reloading it entirely). In a SPA, the page stays the same and just responds to data it receives from the web server (rather than receiving entire web pages from the web server)
Whenever you hear " React.js" or "Vue.js" or even " Angular.js", they're talking about modern JS Frameworks used to create SPAs.
Whenever you hear "jQuery" they're probably talking about server pages (jQuery actually offers no added functionality in 2021; vanillaJS with the advent of querySelectorAll basically does everything jQuery does... except it doesn't
use a stupid
$ API, and it has ES6 Promises)