Stack is the order of your ships.
When one says "that guy is good in stacking", it means the person is good in foreseeing the opponent`s stack and adjusts his own accordingly to win the battle.
For example, I know when a Marauder`s PR suddenly raises by millions he is building either Pyth or Angel, both big ships with good build rate, so I attack with Hercules and win the battle.
Build rate is Power/Turn. I can build 2 Hercules per turn, that`s like 91000 power per turn spent.