Free Basics is an app developed by Facebook as part of the Internet.org initiative, whose goal is to bring everyone free, unrestricted access to basic websites such as AccuWeather, BBC News, ESPN, UNICEF, Dictionary.com, and Facebook itself.
Whether you're able to access all these websites for free depends on your country and Internet provider. This means you need a valid phone number, and your service provider has to be part of the initiative as well. If you meet those requirements, you can access all the previously mentioned websites completely free of charge.
Free Basics by Facebook is an app that promotes a very positive idea: that everyone should be able to access the most useful websites on the Internet, whether they have the money to pay for a data plan or not.