A steam header is the main steam supply pipeline, its function is to deliver steam in as high level of quality (dryness) as possible to the general areas where steam is needed.
It is a distribution pipe from a steam source. From it branches laterals to individual users. You can also have a return header that collects the spent low pressure steam and takes it to a steam condenser or other location. There are other headers called condensate headers that collect the condensed steam of users and returns it to the boiler feedwater system.