Someone else has a server and their infrastructure is set up so you can upload a zip of some executable and they'll figure out how to make it run. You don't worry about any details except your code and whatever API is require to be compatible, and they worry about hosting it, making sure it has memory, CPU time, disk space, DB, etc.
Instead of spinning up a classical server like Apache or IIS for what you need, you just write a single function that you can bind to an endpoint and just host that - the rest is abstracted away from you.
Eli5 server less, even on paper...
Someone else has a server and their infrastructure is set up so you can upload a zip of some executable and they'll figure out how to make it run. You don't worry about any details except your code and whatever API is require to be compatible, and they worry about hosting it, making sure it has memory, CPU time, disk space, DB, etc.
Instead of spinning up a classical server like Apache or IIS for what you need, you just write a single function that you can bind to an endpoint and just host that - the rest is abstracted away from you.