They find what movie, show or whatever you're watching and send it to advertisers with all your metadata, so they can match and track you from an ad impression to you visiting their site to get information and buy.

There's a lot of metadata there to match. If you access their site, for example with your phone, from the same internet connection, they can probably match the information from the TV with one of the tracking cookies on your phone, and then keep tracking you in all the commercial "journey".

This has been already discussed around here because this is one of the reasons smart tvs are so cheap right now, because they're being subsidized by advertisers. Some of the advertisers had in their sites information about their tracking capabilities. I'll try to find that link.