auto-lower-mode is an interactive compiled Lisp function in `frame.el'.

(auto-lower-mode &optional ARG)

Toggle whether or not the selected frame should auto-lower.
With a prefix argument ARG, enable Auto Lower mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.

Auto Lower mode does nothing under most window managers, which
switch focus on mouse clicks. It only has an effect if your
window manager switches focus on mouse movement (in which case
you should also change `focus-follows-mouse' to t). Then,
enabling Auto Lower Mode causes any graphical Emacs frame which
loses focus to be automatically lowered.

Note that this minor mode controls Emacs's own auto-lower
feature. Window managers that switch focus on mouse movement
often have their own features for raising or lowering frames.