The Jargon File (version 4.4.7, 29 Dec 2003):

munching squares
 n.

    A display hack dating back to the PDP-1 (ca. 1962, reportedly discovered
    by Jackson Wright), which employs a trivial computation (repeatedly
    plotting the graph Y = X XOR T for successive values of T ? see HAKMEM
    items 146--148) to produce an impressive display of moving and growing
    squares that devour the screen. The initial value of T is treated as a
    parameter, which, when well-chosen, can produce amazing effects. Some of
    these, later (re)discovered on the LISP machine, have been christened
    munching triangles (try AND for XOR and toggling points instead of plotting
    them), munching w's, and munching mazes. More generally, suppose a graphics
    program produces an impressive and ever-changing display of some basic
    form, foo, on a display terminal, and does it using a relatively simple
    program; then the program (or the resulting display) is likely to be
    referred to as munching foos. [This is a good example of the use of the
    word foo as a metasyntactic variable.]


The Free On-line Dictionary of Computing (30 December 2018):

munching squares

   A display hack dating back to the PDP-1 (ca. 1962,
   reportedly discovered by Jackson Wright), which employs a
   trivial computation (repeatedly plotting the graph Y = X XOR T
   for successive values of T - see HAKMEM items 146--148) to
   produce an impressive display of moving and growing squares
   that devour the screen.  The initial value of T is treated as
   a parameter, which, when well-chosen, can produce amazing
   effects.  Some of these, later (re)discovered on the LISP
   Machine, have been christened "munching triangles" (try AND
   for XOR and toggling points instead of plotting them),
   "munching w's", and "munching mazes".  More generally, suppose
   a graphics program produces an impressive and ever-changing
   display of some basic form, foo, on a display terminal, and
   does it using a relatively simple program; then the program
   (or the resulting display) is likely to be referred to as
   "munching foos".  [This is a good example of the use of the
   word foo as a metasyntactic variable.]