Blizzard introduced a new keyword (like Battlecry or Deathrattle) for the first time ever. It’s called Inspire and it activates whenever you use your Hero Power.
The first example Blizzard has given is Lowly Squire – a 1/2 minion for 1 mana with Inspire: Gain +1 Attack. It means that whenever you use Hero Power, the Lowly Squire gets one point of attack. The effect is permanent and can stack.