@judyannecraig @ajroach42
For not only Judy-Anne:
The part of the instance address after the second "@" is a domain name, i.e., enough of a URL to type it into a browser address bar and get to the instances main web page. The "top level domain" is after the period in the part after the "@". "So ".social" is like ".com" or ".org" or ".edu".

Andrew has been using ".social" as shorthand for "mastodon.social". "mastodon" is a lot to type when you're faced with doing it dozens of times. :)