A native app is a program that is compiled to machine code capable of being executed directly by the operating system. (iOS app, android app, windows app, etc..)

A web app is written in a language that is interpreted (run) by a browser. The browser is the native app in that situation, the web app is “foreign” and is translated to run by the browser.

Typically native apps are faster, and more energy efficient by cutting out the translation step. Developer skills plays a role in how big the difference is.