This function sets the value of *profiler-threshold* below which symbols are not profiled in a repeated profiling run. After a profiling run, all the symbols being profiled have a percentage value for the amount of time they were on the top of the stack. If *profiler-threshold* is set to 40 then by running reset-profiler with argument
:top all symbols which are found on the top of the stack less than forty percent of the time are removed from the list of those symbols considered for profiling.
LispWorks User Guide and Reference Manual - 13 Feb 2015