In MAXScript, the Listener Window is a command-line window that lets you type in and execute MAXScript commands interactively. In other words, you run the script as you go along, rather than running a pre-saved text file.
You open the listener window using the MAXScript Listener command on the MAXScript menu or from the Utilities  MAXScript
 MAXScript  MAXScript rollout
 MAXScript rollout  Open Listener command.
 Open Listener command.