The menubar is the application's top-level menu, normally displayed horizontally at the top of the main frame.
Menubars should normally (since Tk 8.0 onwards) be created through configuring a menu to be in that role using the -menu option to the toplevel for which a menubar is to be installed. The way in which that will work depends on the platform.
tklib has a menubar package with a convenient syntax for specifying menus. See the manual: examples/menubar/demo and menubar(n) .tcl in the repository (note: link is to 20140716 version of demo.tcl).
incr tcl's iwidgets has another menubar package, documented here .
There are a couple of lightweight wrappers for specifying menus floating around the wiki as well: m+ is a nice example, and the menu page has links to more.