what does "host bridge" mean?

From: John Black (black_at_eed.com)
Date: 08/10/04 

Date: Tue, 10 Aug 2004 13:03:18 -0600

I am looking into some PCI document for the development board I have, in
PCI section it mentions a lot a word, "host bridge", what exactly does
this mean? Google only gives me a bunch of products, I want to know
where I can find the terminlogy explanation.

Thanks.