A startup advisory board is a collection of mentors. You'd build the collection in a way that advisors fill in the areas where founders find weakest. For example, the founding team has good experience in marketinga nd PR, but little experience in product development, they may find an experienced product specialist or technical expert to join their advisory board.
Note that an advisory board is not the same as board of directors. Any company can have an advisory board and it is not legally required. Sometimes, investors request to be directors on a board so that they can monitor company performance, but they may or may not serve as advisors.