A web app—also known as “progressive web app” (PWA)—is essentially an application that runs in a web browser. It looks and feels like a native app you’d install from the Play Store or App Store, but it runs in Chrome, Edge, or Safari. You can even “install” the web app to your home screen and launch it like any other app.
There’s a difference between a web app and a mobile website. Many websites have versions specifically for mobile browsers, but they aren’t necessarily web apps. A website has mostly static content, whereas a web app is designed more for user interaction.
A website can be bookmarked and added to your home screen, but it will just open a new tab in the browser when you launch it. A web app opens in its own instance, separate from the web browser it’s running on. That’s why it feels closer to a native app.
One of the big downsides, originally with web apps, were no notifications (a feature of native apps). It seems that this largely works now. Chrome, Edge, and Safari support web apps and notifications.
Find out more, as well as how to install them, at https://www.howtogeek.com/878991/why-you-should-be-using-more-web-apps-on-your-phone/