Profiling is, in a nutshell, assessing how quickly the various parts of your program run. Most often, you'll want your program to run fast, and using a profiler to find slow spots can help you decide what needs work. There is a simple profiler package included in tcllib: http://tcllib.sourceforge.net/doc/profiler.html
Profiling Tcl has more on the subject.
tprof : A profiler for TCL scripts. Allows creation of profiling data sets and later analysis through a graphical frontend.
Sage is a profiling tool for Tcl/Tk programs. It can collect and report data pertaining to where time is spent in your programs.
Paradyn is a performance measurement tool for parallel and distributed programs. Paradyn uses several novel technologies so that it scales to long running programs (hours or days) and large (thousand node) systems, and automates much of the search for performance bottlenecks. It can provide precise performance data down to the procedure and statement level.
Paradyn uses Tcl/Tk for scripting and GUI.