How is ELO determined? Delta?

I get that ELO has to do with a person’s experience level, but what PRECISELY does it mean? How is it determined? What are the minimum and maximum possible? And what is the Delta score? (It shows up if you click on the ELO of a player from the game stats page at the end of a game.)

The admins explained it here:
https://forum.catanuniverse.com/topic/2367/elohowdoesitwork

That explains ELO. Thank you! Does anyone know what the “Delta” score is? My guess is that it’s similarly calculus based and has to do with the percentage of change in a players recent ELO, but I really have no idea.