I understood it as not a wrapped web application, like Electron or Tauri based applications.
Using the same platform-specific graphics API the OS vendor builds their GUI apps with, ideally, but I'll also settle for "not a TUI, not a web application shipped as a desktop app, even if the OS vendor currently builds their GUI apps as web applications shipped as desktop apps".
A GUI that uses native controls and platform UI conventions with the native behavior expected on the given platform, or a near-indistinguishable equivalent of that.