It means 'Salutations unto Śiva the auspicious one, unto Śivatara the one than whom none more auspicious can exist'. Om Namah Shivaya mantra appears without OM in eighth hymn of Namakam(TS 4.5.8.1) as Namah shivaya ca shivataraya (Sanskrit: नमः शिवाय च शिवतराय च IAST: Namaḥ śivāya ca śivatarāya ca). Name of both chapters are Namakam (chapter five) and Chamakam (chapter seven) respectively. Each chapter consist of eleven anuvaka or hymns.
Shri Rudram hymn is taken from two chapters in fourth book of Taittiriya Samhita (TS 4.5, 4.7) of Krishna Yajurveda. This mantra is present in the Shri Rudram hymn which is part of the Krishna Yajurveda.
