Jiuquan 九泉 [nineth spring] refers to the place where the dead are buried, that is, in the underworld. Because when digging deep into the ground, yellow spring water will come out. So traditionally, the term huagnquan 黄泉 [yellow spring] refers to the underground and nether world. As for the number jiu 九 [nine], it is not necessarily a number but a general term for many. So, jiuquan literally means the place deep under the ground.
The term huangquan famously appears in “Earl of Zheng overcame Duan at Yan” 鄭伯克段於鄢from Zuozhuan 左傳 [Zuo Tradition], where Earl of Zheng vowed to never see his mother who betrayed him again: “We will never see each other until we reached the yellow spring!” 不及黃泉，無相見也.
Daoism later developed the concept of jiuquan into nine springs that governs the jail and punishment in the nether world, see https://zh.wikipedia.org/wiki/%E9%BB%83%E6%B3%89.
