Narsimha (Combination of Nara-Simha: man and lion) is considered to be an Avatar of Lord Vishnu. He had the unique feature of combining two physical forms–of a man and of a lion. He incarnated to punish the mighty King Hrinyakshipu, who was continuously torturing his own son, Prahlada.
Here is the Sanskrit mantra and the Roman transliteration for those who can’t read the Devanagari script, but would like to understand the proper pronunciation of the mantra.
Here is the meaning of this mantra in both Hindi (meaning sourced from WikiPedia) and English languages:
(हे क्रुद्ध एवं शूर-वीर महाविष्णु, तुम्हारी ज्वाला एवं ताप चतुर्दिक फैला हुआ है। हे नरसिंहदेव, तुम्हारा चेहरा सर्वव्यापी है, तुम मृत्यु के भी यम हो और मैं तुम्हारे समक्ष आत्मसमर्पण करता हूँ।)
(O Angry and valorous Mahavishnu, your flames and heat is spread in all the directions, and your face is everywhere. O Narsimha, the intense and auspicious one, you are the death of the very death itself–I bow to you)