Utility procedures for managing the input focus.
http://purl.org/tcl/home/man/tcl8.4/TkCmd/focusNext.htm
SYNOPSIS
tk_focusFollowsMouse
DESCRIPTION
tk_focusFollowsMouse changes the focus model for the application to an implicit one where the window under the mouse gets the focus. After this procedure is called, whenever the mouse enters a window Tk will automatically give it the input focus. The focus command may be used to move the focus to a window other than the one under the mouse, but as soon as the mouse moves into a new window the focus will jump to that window. Note: at present there is no built-in support for returning the application to an explicit focus model; to do this you'll have to write a script that deletes the bindings created by tk_focusFollowsMouse.
See also:
Tk syntax help - Arts and Crafts of Tcl-Tk Programming - Category Command, part of the Tk package.