In T13 the Incarna defines the thing that a Descendant or Character is made from. Everything that exists (and some things that don't) have an Incarna. Each Facet has its own Incarna type. The Facet you have picked as your Incarna Facet is also known as your Formation Facet. Incarna can affect all manner of things about the Character and Descendants, but rarely requires any special mechanics or rules.