In the passage, men and God both have some way to offer water. God has a spring, while man has a cistern. Being a desert setting, the need for water cannot be taken for granted.
Let's first look at the cistern of man. This is a reservoir for water that has to be dug, hewn from the land underground. The work of men is to "build" something that will hopefully meet their need for water. The result is artificial. It is a container which tries to set boundaries and store up. It may work for a while, but being artificial, it cannot last. All of the works of men - be them machines or buildings - will deteriorate someday. Even in this passage, they are already faulty and broken.
On the other hand, God's creation is always renewing itself.
The water God has bubbles from an eternal spring. It is never to run dry because it replenishes itself. While men work to store up treasures and lose them, God gives freely and His children are not in want.
Men store and God gives.