Basically this is showing the zones our hot-water-based radiant heat will be divided into. The manifold is where all the plastic pex pipes running the hot water to the radiators and radiant floors start and end and where you turn on/off certain zones or the whole system. One benefit of such a complicated looking system is efficiency- you can control the amount of water, and therefore heat, going to each zone. Hopefully this means a balanced heating system- no room would ever be too hot while other rooms are chilly.