I think they're saying - "Make sure you always use local URL paths for local resources. If you refer to a resource on your server via an external URL [why one would want to do that, I don't know], you're going to trigger an HTTP request which will decrease your web page performance."

URLs are "uniform resource locations" and can refer to local or remote resources. That's why they're "uniform". At least that's my understanding.

I wouldn't think too hard about this. It's just poorly worded.

I think, anyway. ;-)

HTH a bit.

Cheers,
Scott.