A program installed on a computer that opens from the desktop or Start menu in its own window, rather than inside a web browser.
A desktop app is software you install on a computer. It opens in its own window, has menus and keyboard shortcuts, can keep working without internet, and talks directly to devices connected to the PC such as printers and barcode scanners. Cashier, accounting and inventory programs in shops are familiar examples.
It matters because many small shops and offices still run on one counter computer and a connection they cannot always rely on. A desktop app is the natural fit there, while a web app suits teams spread across several places. In Fanann Windows Studio you describe the program, try it in a Windows-style window, then download an installer for Windows 10 or 11 (64-bit). Practical tip: decide first who will use the program and from where. One computer at the counter points to a desktop app; staff in several branches may need shared cloud data or a web app.