This is the most famous Shiva mantra – om namah shivaya. Also known as the Panchakshara, or the “five-syllable” mantra (not counting om). It calls out to Shiva, the compassionate Deity of destruction, with reverence. How compassion & destruction just went in the same sentence is explained by approaching Shiva as the god who can help you destroy your material body/mind based ego, and connect with your higher self through meditation.
Shiva literally means ‘auspicious, benign, friendly’. Namah means ‘offering respects, bowing down, worshipping’. Om is the sacred cosmic syllable that is often invoked in the beginning of mantras – read more about om’s meaning here.