It means: people are in the rivers and lakes, they can't help themselves, they can't stay out of it, they are detached from the rivers and lakes.
The appellation of Jianghu was first put forward by Zhuangzi, who came from a great master, Zhuangzi. The original sentence is "spring is dry, fish is on land, it is better to forget each other than rivers and lakes."
After the spring dried up, the two fish did not leave in time and were trapped in a small depression on land. Two fish can't move, so they wet each other with foam to keep each other wet. At this time, the two fish will think of the free and unfamiliar life in the past.