User-defined commands are pre-defined commands that you create yourself. Effectively it's like creating a toolbar button with your own function on it, but instead of the button living on a toolbar, it lives in a global command list and other buttons or menus can refer to it by name. User-defined commands are created via the User Commands tab of the Customize dialog.

When you create a user command you give it a name (which must be unique and not clash with any of the Opus commands), and other buttons can then run your user command using its name. User commands can also accept parameters (arguments) on the command line, which lets you pass through parameters to external programs or scripts that your user command might invoke.

See the documentation for the User Commands tab for more details on creating and using user commands.