Trace evaluates an expression and displays the result in the Output panel in test mode.

When you publish a flash SWF through Test Movie, the file is opened in the Flash authoring tool and the value of the expression parameter is displayed in the Output panel.

Use this action to record programming notes or to display messages in the Output panel while testing a movie. Use the expression parameter to check if a condition exists, or to display values in the Output panel.

You can use the Omit Trace Actions command in Publish Settings to remove trace() actions from the exported SWF file.

trace()

ex.
n=1;
trace ("the value of n is " + n);