Web apps are not real applications; they are really websites that, in many ways, look and feel like native applications, but are not implemented as such. They are run by a browser and typically written in HTML5. Users first access them as they would access any web page: they navigate to a special URL and then have the option of “installing” them on their home screen by creating a bookmark to that page.
A web app is best for products that require minimal native gestures and don´t require acess to features from mobile devices like push notifications. However, it won´t feel like an native app and cannot be distributed through native app stores.
Moodboard is an example of a web app.