This tool opens a file or URL with the default application associated with it on the user's operating system:
- On macOS, it's equivalent to the `open` command
- On Windows, it's equivalent to `start`
- On Linux, it uses something like `xdg-open`, `gio open`, `gnome-open`, `kde-open`, `wslview` as appropriate

For example, it can open a web browser with a URL, open a PDF file with the default PDF viewer, etc.