git: What does the name mean after the colon in the ssh address of GItHub?

For example,

git@github.com: JohnCoates / Aerial.git

in this command, I think

git: username on the GitHub Unix server
github.com: hostname
/Aerial.git: directory

.

So what is it : JohnCoates? In general, there would be a port number, but JohnCoatesIt does not look like a port number.