A web app is a program, usually delivered in the form of an installable piece of software on an end-user device that does something whether it be booking an Uber taxi, measuring your heartbeat or teaching kids how to read. The web app’s core logic stored on a server in a remote location and the user sees on the client side, the result of their interaction with the web app.
For example…
Take your email inbox. Most providers will allow you to check your mail through an app or a web-based interface, allowing you to access your email on any device which has the feature installed. You will still be downloading content from the mail server where your account is hosted and sending email will perform the same function as if you were to do it on the native app.