By default, ads are blocked, but users of Brave can opt-in to have privacy-friendly ads from the Brave network displayed on websites they browse.

Brave get 30% of the revenue from those ads, and the users get 70% of the revenue as BAT tokens

Users can then use those BAT tokens to tip the websites they like that signed up as Brave publishers (see a list here: https://batgrowth.com/publishers)