BAT simply means Basic Attention Token.
BAT should radically improve the efficiency of digital advertising by creating a new token that can be exchanged between publishers, advertisers and users. It all happens on the Ethereum blockchain which is already well known.
To make it possible 3 main stages has to be introduced into real life:

Brave Browser (already done)
Brave is a fast, open source, privacy-focused browser that blocks malvertisements and trackers. Brave aims to transform the online ad ecosystem with micropayments and a new revenue-sharing solution to give users and publishers a better deal, where fast, safe browsing is the path to a brighter future for the open web.

Basic Attention Token (will be available soon)
The Basic Attention Token can be used to obtain a variety of advertising and attention-based services on the Brave platform, as it is exchanged between publishers, advertisers and users.

Brave + BAT = A New Deal (nearest futere)
The Brave browser knows where users spend their time, making it the perfect tool to calculate and reward publishers with BATs. This service creates a transparent and efficient Blockchain-based digital advertising market.