a web app that uses modern web capabilities to deliver an app-like experience to users. They are installable and live on the user's home screen, without the need for an app store.
people visit the web from their mobile devices, but only a few keep returning. Most people visit the web in passing, this means that it is not their primary means of accessing the web.
(such as name, author, icon, and description) in a JSON text file. The purpose of the manifest is to install web applications to the homescreen of a device, providing users with quicker access and a richer experience. - Mozilla (mzl.la/2lefIbe)