Namah Shivaya means “O salutations to the auspicious one!”, or “adoration to Lord Shiva”. It is called Siva Panchakshara.
Panchakshara meaning the “five-syllable” mantra (viz., excluding the Om) and is dedicated to Shiva.
This Mantra appears as ‘Na’ ‘Ma’ ‘Śi’ ‘Vā’ and ‘Ya’ in the Shri Rudram Chamakam which is a part of the Krishna YajurvedaThe mantra without the initial Om was originally a verse in the eighth hymn of the Namakam section of the Shri Rudram, (TS 4.5.8.1) itself taken from the Taittirīya Samhita, a recension of the Black Yajurveda. It appears as, Namaḥ śivāya ca śivatarāya ca (Sanskrit: नमः शिवाय च शिवतराय च). The English translation of the mantra is, “Salutation to the auspicious one and to the more auspicious”.
This mantra also appears in the Rudrashtadhyayi, a part of the Shukla Yajurveda. In the Rudrashtadhyayi, it appears in the 5th chapter (also known as Namakam) verse 41. and also in the Rudrashtadhyayi which is a part of the Shukla Yajurveda.