GNU Emacs Manual
AD.9.2 Recursive Editing Levels
Recursive editing levels are important and useful features of Emacs, but
they can seem like malfunctions to the user who does not understand them.
If the mode line has square brackets `[...]' around the parentheses
that contain the names of the major and minor modes, you have entered a
recursive editing level. If you did not do this on purpose, or if you
don't understand what that means, you should just get out of the recursive
editing level. To do so, type M-x top-level. This is called getting
back to top level. See section AC.26 Recursive Editing Levels.