An output feed is a generated URL containing all of your product information plus the new treatments you've created.
Only one output feed is needed per catalog you manage. To create an output feed, go to the Output Feeds tab at the top. Then, click New Output Feed.
You'll be able to name your output feed and select if you'd like it to be a Single Treatment or a Multi-Treatment feed.
After you click Create, you'll be able to start managing your new output feed!