Here's a trailer showing off the season pass for Assassin's Creed: Unity. The season pass will include some more missions set in Paris, as well as a darker story set in 1794 Saint-Denis, that will follow on from the events of the main game. Also included in the season pass will be some additional weapons and items to customise your character.
The most exciting season pass content has almost nothing to do with Unity, or revolutionary France. Assassin's Creed Chronicles: China is a 2.5D, side-scroller set in 16th century China. This means that technically there will be three new additions to the Assassin's Creed franchise this year. I'm really hoping Chronicles will not be a new gen. exclusive, and would definitely buy this if it were released as a stand alone download for either PC or PlayStation 3. Hopefully Chronicles' will also open the door for the development of a full game set in a similar time period, covering some more historical events in China.
You can also pre-order the season pass to get extra stuff, which I'd be rather weary of doing, since the main game hasn't even been released yet. Each to their own I guess.