What Is a Feeder Site?

In episode 109 of the weekly Hump Day Hangouts by Semantic Mastery, one viewer asked about feeder sites.

The exact question was:

Hi guys!

Folllowing up on my question of last week about WordPress and Google Sites. Bradley talked about a “”feeder site””. This is a new concept for me can you explain a little bit or lead me to some ressources explaining this.

Thanks,
Keep up the good work!