The fae are divided into two Courts: Seelie and Unseelie. The Seelie Court see themselves as the guardians of tradition, and act as chivalrous peacekeepers and protectors of the weak. Honour, loyalty, virtue, justice, beauty, and order are very important. The Unseelie Court is the opposite: they love nothing better than to mock tradition. They want to be wild and free, and see the traditions and chivalrous codes of the Seelie Court as too restricting & too outdated. They are rebellious, and love change.
In Arcadia the Court of your Character is in most cases determined by what Abilities you pick, and not very often does your Court determine your game. Sometimes Leagues tell you you can recover X when you are Seelie but Y when you are Unseelie, and sometimes a Waylay will engage with you differently depending on your Court. But mostly it seems irrelevant, I think, to most players.
These two Courts are very essential in the world of the fae, though, and in Changeling: The Dreaming they are taken very seriously, because your Court will determine how you will act and react.
Therefore, if you want to introduce some role-playing in your Arcadia games, this is a good place to start: your Court defines the way you play, what you do, and how you deal with the other player and even Waylays.
The Changeling handbook describes the two Courts in a few pages, but a really good summary is given in the two boxes below (on page 73):