A Rattlebox (or Doombox) is simply a method to roll a lot of sets of dice, rapidly. It is typically a Plano organizer with 36 compartments, that has 2D6 in each well, and is named by the rattling sound it makes as you shake it.
In Classic Battletech these boxes are typically used to resolve hit locations for cluster typer weapons (LRM, SRM, LBX AC, HAG, MPOD) or can also be used to resolve multiple to hit rolls at the same time for equipment that has a lot of the same weapons (Piranha, Nova (Black Hawk) Prime, or Komodo come to mind). Using a Doombox can greatly speed up play in a large engagement.
The biggest problems that I have with my Doombox, is that I always forget the thing at home, and it still takes a while to resolve damage, because I don't have the hit location tables completely memorized and I'm always referring back to them. The biggest problem other people in the play area have with a Rattlebox, is that that thing is just loud. Really, really loud. You're shaking 72 dice in an enclosed hard plastic box, it's gonna, well, rattle.
So, I made a digital one.