"Balloon help" is sometimes known as "tooltips".
Effective Tcl's example code, [L1 ] as well as Jeff Hobbes's widget library [L2 ] include a balloonhelp.tcl
[Where is its documentation?]
Bwidgets have balloons under the title of "Dynamic help", here's an example how simple that can be (RS):
interp alias {} help {} DynamicHelp::register
Button .f.open -image $im(open) -command {starDOM::openFile .t}
help .f.open balloon "Open existing XML file"[Also describe the section in the M&M book.]