Activity Stream

I'm having trouble with getting my Activity Stream to show. Part of the problem is that I'm dense and words on a page are not immediately evident to me. So, for the life of me, I can't figure out what is this suggestion is for?

Enter a comma separated list of domains to show activity for, or leave blank to use current domain.

I am expecting FB activity on my site. I don't understand what listing domains has to do with anything. I'm thinking this widget would just display whatever activity I have on my Facebook wall, and it has nothing to do with any other domain, other than FB. Please illuminate.