It actually refers to where the centre lines of brake calipers arm are, in plan view.
If you look at the Monoplanar brakes, where the two "halves" of the caliper are interleaved, you'll see that the brake-blocks are exactly aligned across the width of the rim and both are "stepped", in plan view, the same distance away from the centreline of the caliper arm.
Most brakes (including all of the current Campagnolo designs of brake), one brake arm is in effect, "in front of" the other, so one caliper arm itself is "stepped" more than the other ... otherwise, the two blocks would not meet the rim exactly opposite one another, the offset L/R would be the same as the depth of the caliper arm at the pivot.
Monoplanar was / is just a term, "in a single plane" - more common would be to refer to a number of points being co-planar (in the same plane), i.e. three points define a plane so the feet of a tripod are said to be co-planar ... but I suppose one could also say monoplanar, which has the merit of being distinctive!
Monoplanar is probably not commonly used because first, co-planar is just as comprehensible and second (maybe I am reaching here), there's potential in spoken English, to confuse monoplanar and monoplane.
I always thought they were very elegant brakes though ...