Most content on a screen is fixed once you make it: an image, a video. An app is the opposite. It is a small program that shows information which keeps changing on its own, a piece of dynamic content rather than a still one.
A dashboard that refreshes through the day, a calendar that fills with the day’s meetings, a weather panel, a news feed: each is an app. You set it up once, and it keeps itself current, pulling in whatever it is meant to show without anyone touching it.
There is a second thing that sets an app apart, beyond keeping itself current: it is built for the screen. A web page is made for a person at a laptop with a mouse, with small text, menus, and a layout that assumes someone is sitting close and clicking around. On a display people read from across a room, that rarely works well.
An app is designed to be shown rather than used. It is laid out to fill the display and to be read at a distance, and it runs on its own with nothing to click. It also comes packaged for the job: you give it a few details, which dashboard, which calendar, which city, and it knows how to present them on a screen.
So a web page is something you point a screen at and hope it fits; an app is built for the screen from the start.