A bow has a certain feel to it. a good bow pulls smoothly and at the end of the draw, the weight on your drawing hand stays pretty constant. When a bow "stacks", it seems to feel stiff at the end of the draw, and every inch more pull increases the weight more than the previous inch, and feels really heavy to hold.
I know there is a better way to describe "stacking", but that is about the best I can do.
This does not mean that the bow wil lbe unshootable, especially if you have a shorter than normal draw.