A Stronghold is a defensive position, such as a fortress or castle, turned hold. They are typically bolstered by defensive walls and other emplacements.
While within the Stronghold, you gain a number of benefits.
You gain a +1 bonus to AC while you are behind cover.
You gain a +1 bonus to Wisdom and Charisma saving throws.
If your Stronghold is making profits from combat, military, or mercenary matters, they do so notably better than other types of holds, typically translating as an increase in potential profits.