"Serverless" in this instance means exactly what it does for other platforms. It is basically a marketing gimmick. It doesn't really mean no server it means server"LESS". I.e. less server. Instead of having a dedicated server that is running your backend (or in this case the postgres sql database) 24/7 . When someone makes the request that needs your backend (in Svelte would be your +page.server files) it loads up your code and runs it then listens for about 5 more mins and if nothing else comes in it goes back to sleep. This allowed all these companies with servers sitting around to launch these new service levels at a fraction of what it cost to have a dedicated server. By being able to slice up the resources and sell to multiple people they actually make more money, people that otherwise couldn't afford a dedicated server can now get their apps online. It is mostly a win win except for the confusion it causes and sometimes there are limitations of course. Lastly, when you are hosting your app on vercel your using serverless for your backend (all your +page.server), and if you are using any of their storage options you are using serverless for them as well. I hope this helps! If you have any other questions let me know!
