English isn't my first language either, so don't feel bad asking me to explain.
The "old" standard was to have parallel top and bottom lines, lets still use this as my ideal. If the flank is "cut up" or high, it means the animal is shallower at the flank than it is further forward towards the front of the animal.
High flanks usually indicates later maturing animals that will mature at a taller mature size. Its animals that typically continue to grow for longer without putting much fat on. This trait was associated with the Continentals of yesteryear and the British breeds foolishly pursued this at all cost, resulting in losing some traits that made them popular in the first place.
A low flank indicates an animal who is earlier maturing, who will more readily put fat on and is usually easier doing as a result. It may not be the ideal feedlot animal for the feeder who makes money by selling more feed, but it is the ideal grazing animal for a low input system under harsher conditions.