Mark the current pane (see select-pane -m).
So that means you've pressed
C-b m (or equivalent in your environment) to mark that pane.
Marking a pane is useful for certain pane operations; from the section on
The marked pane is the default target for
To clear the mark, press
C-b M. If you have
M mapped to something else already, you can also use
C-b : (to enter the
tmux command prompt), followed by
select-pane -M to clear the mark.