If an overwhelming number of sites of similar subject matter link to the same site, it is considered an "authority" --

The reasoning is that - if sites of similar subject matter are all linking to the same site - they must respect it for some reason.  Therefore, it is an "authority" on that specific topic. 

As real estate agents, you determine the "authorities" for real estate.  Choose carefully who you link to.

Links from "authorities" are more important than links from other sites -- and becoming even more important.