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.
etprof is a little profiler in pure-Tcl.
Mark Smith's profiler.tcl, [L1 ], is yet another pure-Tcl variation.