The lord was sitting on a mattress of straw because such a sitting place is accepted by persons who are practicing austerities to gain understanding of the Absolute Truth. In this verse it is specifically mentioned that he was speaking to the great sage Nārada, a celebrated devotee. Nārada was asking Lord Śiva about devotional service, and Śiva, being the topmost Vaiṣṇava, was instructing him. In other words, Lord Śiva and Nārada were discussing the knowledge of the Veda, but it is to be understood that the subject matter was devotional service. Another point in this connection is that Lord Śiva is the supreme instructor and the great sage Nārada is the supreme audience. Therefore the supreme subject matter of Vedic knowledge is bhakti, or devotional service.