Tauri is an open-source framework (a set of libraries and tools that an application is built on) for apps on Linux, macOS, Windows, Android and iOS, all from a single codebase. It is maintained by the Tauri Programme within The Commons Conservancy.
The user interface is written with any frontend framework that compiles to HTML, JavaScript and CSS. The application logic is written in Rust, and Swift and Kotlin bindings give deeper access to the system. The frontend calls Rust functions through an invoke function. A Tauri app does not bundle a browser engine: it uses the webview (the web page renderer that the operating system already has), so a minimal app can be less than 600 KB. Two libraries maintained by Tauri do the low-level work: TAO creates the windows and WRY renders the web view. Official plugins add features such as a system tray, notifications, file system access and an updater, and each major and minor release has a security audit.
Tauri is free and open source, under the MIT license or MIT/Apache 2.0 where applicable.