What is the fundamental essence of finance?

A 3-minute quick logic breakdown, explained simply:
Understand a simple logical story, and you'll see through the core underlying logic of the economy and finance.
Set up three people: planting apples, making clothes, fishing, none of them have a penny.
The tailor makes clothes for the fruit farmer, the fisherman supplies the tailor, and the fruit farmer delivers fruit to the fisherman.
The three owe each other on credit, each owing 2 yuan, for a total debt of 6 yuan.