The documentation for the [[rename]] command is available at [http://tcl.activestate.com/man/tcl8.3.2/TclCmd/rename.htm].

The [[rename]] command is most often used for [wrapping commands].  In other words, the programmer uses [[rename]] to hide another command -- so that a wrapper can be placed around the hidden command to change its behavior.  Examples of this important concept abound on the Wiki;
among the clearest are:
[DGP]: Be aware that if you `rename` a command into another
   * [Overloading widgets]
   * [Text variable for text widget]
   * [Forward compatibility]