Airtel Zero plan is introduced by Airtel where user can see some websites without having any internet plan or data recharge. The data which is used by the user to browse such websites are paid by the respective company to Airtel.
Ex: You can access Flipkart website as well as Flipkart app on your mobile without any data plan. If you are using Airtel network you can access Flipkkart App, browse product, buy product without any internet plan. The internet which you use to browse Flipkart App is paid by Flipkart to Airtel.