An establishment is a building of commerce, a business, a tavern, or maybe all of the above. In the end, its primary purpose is to bring in profits, and attract new faces.
While within the Establishment, you gain a number of benefits.
You gain a +2 bonus to any skill check made for purpose of doing trade, or valuing items.
If your establishment is making profits, they do so notably better than other types of holds, typically translating as an increase in potential profits.