On this page of the Valheim game guide, you will learn how to obtain dark wood, one of the materials added to the game in the Hearth and Home update.
Dark wood, unlike other types of wood, isn't a resource that can be collected while exploring and then stored in your inventory. You will only see this resource once you have built a given structure.
Dark wood is a combination of two resources that need to be obtained separately - regular or fine wood and tar. Once you have both resources, new blueprints will be unlocked in the building panel.
Dark wood structures mainly serve as decorations and aren't required to create a functioning base. However, they might add some character to your base.