The suffix 性 (-xìng) turns adjectives into abstract nouns: 必要 'necessary' to 必要性 'necessity'. Same pattern: 重要 to 重要性 'importance', 可能 to 可能性 'possibility'.
usage
Common collocations: 强调…的必要性 (emphasize the necessity of), 认识到…的必要性 (recognize the necessity of). It's the formal/written cousin of saying 必须 'must'.
