Before you can configure Media Cloud to work with imgix, you'll first need to setup a source on imgix. That's a bit beyond the scope of this documentation, but imgix's own documentation is very thorough and easy to get through.
Once you've created a source in imgix, add the domains you've setup in the source here.
The reason you can add multiple domains was to support domain sharding which in the good 'ol HTTP/1.1 days helped improve performance on image heavy websites. But imgix now uses HTTP/2 which makes domain sharding irrelevant. We only allow multiple domains to be specified for backwards compatibility.
imgix Signing Key
It's highly recommended that you set up image signing. You can read more about it here.
Once you've enabled Secure URLs in your imgix source, copy the token from imgix and paste it in here.
This should always be turned on and I can't fathom why you'd want it turned off. But in whatever weird edge case you've found yourself tangled up in where you need to serve images from imgix over http, you can disable https here.