To make a mousse, you need two parts: a base (which provides the flavoring) and an aerator (which makes the mousse light and fluffy).  The base can be anything from melted chocolate to puréed fruit, while the aerators are usually whipped cream, meringue, or eggs.  In this recipe, we're using maple syrup as our base, and whipped cream as the aerator.  Read more about bases and aerators if you're curious.