Can someone here maybe tell me something about that?

I've come across this term in a poem by Li Bai that we have to analyze for a class (not easy at all after only two years of Chinese).

I think I found a definition on zdic.net, but I can't really make sense of it.

Here it is:
《史记·张耳陈馀列传》：“ 汉 八年，上从 东垣 还，过 赵 ， 贯高 等乃壁人 柏人 ，要之置厕。上过欲宿，心动，问曰：‘县名为何？’曰：‘ 柏人 。’‘柏人者，迫於人也！’不宿而去。”后遂用为皇帝行止戒备的典故。 唐 李白 《枯鱼过河泣》诗：“万乘慎出入， 柏人 以为诫。”

I'm pretty sure that's the correct definition, since the second reference in this definition is actually the poem I'm talking about, but I really can't understand it...

It would be great if you guys could help me out. :)

According to zdic, 柏人 is 1) the name of a place now in 河北省 which belonged to the state of 赵 during the Warring states period (战国 属 赵), or 2) an allusion to the precept that the king should take precaution with his behavior (用为皇帝行止戒备的典故).

The story:
汉 八年，In the eighth year of the Han dynasty
上从 东垣 还，the king(上) while returning(还) from 东垣
过 赵， passed through赵
贯高 等乃壁人 柏人，therefore(乃) a group of person headed by 贯高 (贯高等) hided some persons in the hollow wall (壁人) in the place 柏人
要之置厕。aiming for the king (要之) by the side of some quiet place near the inn (置厕)
上过欲宿，When the king passed through 柏人 (上过), he wanted to stay overnight (欲宿)
心动，But then he thought about something (心动)
问曰：‘县名为何’ and asked, “What is the name of this county?"
曰：‘ 柏人 。’ The answer was柏人
‘柏人者，迫於人也！’ The king said, "柏人 means to injure, devastate people (迫於人)."
不宿而去 So the king did not stay overnight and left

pts,

Just curious, how long have you been studying Chinese?

Thank you so much, pts. :)