This "goal", with no arguments, allows you to exit from Prolog in an operating-system-independent way. Typing the "end-of-file" character (control-D in Unix/Linux systems) will also get you out. Different operating systems may use different end-of-file characters. Example of halt:

% prolog
--- Welcome message from Prolog interpreter ---

?- halt.

The "%" signs in this example represent the operating system command interpreter (aka "shell") prompt, not a Prolog comment.