This refers to a part, or a section, of the full 19x19 go board. It is usually rectangular, and usually in the corner. Sub Boards are often used for carrying out some sort of restricted analysis, or as the basis for a database search.
Example: a 5x10 corner sub-board on which one might analyse joseki, and which will turn out to be too narrow for most joseki.
Probably an alias of Partial Board.