人比黄花瘦

Closing line of 李清照《醉花阴》 (Li Qingzhao, Northern Song, c. early 1100s), sent to her husband 赵明诚 during one of his official postings: 薄雾浓云愁永昼，瑞脑消金兽。佳节又重阳，玉枕纱厨，半夜凉初透。东篱把酒黄昏后，有暗香盈袖。莫道不消魂，帘卷西风，人比黄花瘦 (Thin mist, thick clouds — all day I grieve; sandalwood-incense melts in the bronze beast. The festival comes round again — Double Ninth; jade pillow, gauze curtain — midnight's cold just seeps through. At the east hedge I held wine past dusk; a subtle fragrance filled my sleeve. Don't say the soul isn't undone — the curtain rolls in the west wind, and the person is thinner than the yellow flower). Traditionally called the greatest closing in all Song ci.

黄花 here specifically means 菊花 (chrysanthemum), which blooms at 重阳节 (Double Ninth) when Li Qingzhao composed the ci. 比 is the classical comparative ('compared to / more than'). Always canonically preceded by 帘卷西风 — the image of the wind revealing the thin figure.