It is a symbol of the Placenta for Humanity. It is a Super Natural Sphere object that appears all throughout Grant Morrison's The Invisibles.
The Invisibles as a series is a magical symbol, it is an art form meant to change ones will. Because The Invisibles has a magical intent, it is referred to as a hypersigil. Barbelith works almost like a trigger system to the hypersigil. Once we see Barelith, it is suppose to connect us to that symbolic connection with have with Humanity. It is the Placenta of our reality, it is the connection between humanity and the holographic world that we see.