延年益寿 is a four-character idiom (成语) meaning 'to prolong life and increase longevity' — it describes something that helps you live longer and stay healthy. You'll see it in health and wellness contexts, often describing traditional medicines, tonics, exercises, or diets. The literal translation 'promise longevity' is misleading because it's not about making a promise; it's about the actual effect of extending one's years and adding to one's lifespan.