A custom short domain is the domain that takes the place of "bit.ly" in a shortened URL. If you were to use "myname.me" as a custom domain, instead of appearing like this:
your links would appear like this:
A custom short domain is a way to brand your links across the web! For example, The New York Times uses "nyti.ms" as a custom short domain and TechCrunch uses "tcrn.ch".
Interested in setting up your custom short domain? Learn the steps here.