When you call a function that is traced, LispWorks collects information about the arguments it was called with and the values that it returned. This information is printed to the trace output stream, which might be the Listener or the Background Output. In addition, if a Tracer tool is on the screen, the information is shown in its Output Text view and collected in its Output Data view in a tree format.
LispWorks IDE User Guide (Windows version) - 25 Nov 2011