I suppose the official rules in the updated REG will help, but let me take a stab at explaining the keyword: Unity.
When a card has the Keyword, Unity, something about the card (usually the special ability) is dependent or contingent upon the qualifications that the Unity mechanic is looking for. Basically, a cause and effect situation or if X is true then do Y. (Unity is the cause and the Special Ability is the Effect). Let's look at, "Zion."
The "X" is Human Heroes (Royalty) and "X" cannot be null.
The "Y" is Either
a) If you play this Site, you may take a Hero from deck or discard pile.
b) If your lone hero attacks, you may take a Good OT card from Reserve.
So, When "Zion" enters your territory you choose it's type, if the chosen Type is "Site" you get an "enter" trigger. The game then looks to see if you qualify to do this Special Ability by checking if Unity "X" meets the requirements; if so, you then proceed to the Special Ability "Y."
Similarly, if "Zion" was chosen to be a Fortress when it entered the territory, every time you attack with a Hero the game does two checks to see if Zion will trigger: 1) Is your Hero alone, and 2) if you have Unity; if both criteria are meet, proceed to the Special Ability.
Once again, IM sure the REG will have better wording, but if you think of Unity as If X then Y. It should help you for the most part until we have official wording.
