I think you can? https://contributor.google.com . Not that I’ve tried it - I’m not allowed to in the UK.
Essentially, it's a way for a site's users to bid for its ad spots. If you bid more than the advertisers, you don't have to see the ads.
It's quite a clever idea really, but I don't think it has much uptake from publishers.
It's too bad there wasn't some way to have it cross over networks too.