“Stacking” is more noticeable in DA/SA Revolver’s and Pistol’s and is less noticeable in DAO Revolver’s and Pistol’s. Essentially the trigger is loading up prior to its break or release and gives you a clear indication of when the trigger will release, you can easily pull and hold the trigger up to the release point, catch a breath, relax and shoot. Gun writers sometimes equate a heavy trigger pull with “stacking” but I disagree with this premise, instead I consider the trigger load up to be beneficial.
I personally don't like a trigger that stacks, but some really great shooters like it because stacking allows them to "feel" or "stage" the trigger. So to each his own. It's not necessarily a bad thing.