The ad-tech ecosystem is a bewildering variety of middlemen & complexity (agencies, trading desks, demand-side platforms, network exchanges, yield optimization, rich media vendors etc):

• Users suffer from digital pollution: they have lost their privacy and have a bad user experience. 600M devices now use ad blocking, and it will increase to 1.7B by 2017. 
• Publishers have lost billions in revenue while fraud has skyrocketed. The content that works best is clickbaity. 
• Advertisers face poor reporting and targeting. Facebook & Google are not their friends. 

User attention is anonymously monitored in the Brave browser, a fast open source, privacy-focused browser that blocks invasive ads and trackers, and contains a ledger system that anonymously measures user attention aggregate to accurately reward publishers. 

Brave cuts out all third-party trackers and middle-players, while providing publishers with a substantially larger revenue share than they are receiving in existing marketplaces. Brave thus aims to reset the online ad-based Web ecosystem, giving advertisers, publishers and customers a win-win solution whose components and protocols can become future Web standards.

The high-level concept is of monetizing attention is:

The advertiser sends a payment in token along with ads to users in a locked state Xa. 

As the users view the ads, the flow of payments unlocks: 

• ... keeping part of the payment for their own wallet (Xu) 
• passing on shares of the payment to Brave (Xb) and 
• passing the remainder on to the Publisher (Xa-Xu-Xb).

The token collected can be used to pay for premium access or to change it for ETH.