They are viewers who have joined BrightTALK and subscribed to your channel to see your webcasts. They either came directly from the BrightTALK network (organic), or subscribed after being invited to attend by your marketing efforts.
With all BrightTALK channels, you can see the detailed information about the subscribers you brought to your channel including BrightTALK referral subscribers.
Subscribers who have discovered your webinars via BrightTALK.com are listed in your channel owner 'Insight' tab as:
You can download your subscribers by clicking on the "Insight" tab, scrolling to the bottom of the page, and clicking "generate" next to the report that you'd like to download. Once the report has generated, click on the title to download it.