"Header board" is terminology used in deck construction. When building a deck that juts out from a house, the deck is usually tied into the house structurally. To achieve this, a header board or "ledger" is bolted to the house. The header board is one end of the deck, and the deck at this end derives support from the house. This board will be the same width as the deck joists, which will be "hung" from the header board using joist hangers.
The joists meet the header board at a right angle, and the tops of the joists are at the same level as the top of the ledger. It is important to establish this level properly early on in a deck construction project, since it provides the surface for the floor of the deck -- i.e., the decking.