What does Genesis 2:21 mean?

In verse 18, God declared that He would make a helper for the man, since it was not good for the man to be alone. Then in the following two verses, Adam goes through the process of naming all of the animals God had created. In doing so, he finds out first-hand that none of them can serve as a suitable helper for him.

Even as God creates, He often does so with the intent of modifying that creation later on (Genesis 1:9–12). So now God goes to work, taking action to complete his intended design of the human race. He causes a deep sleep to fall on Adam: the first anesthesia before surgery. While Adam is asleep, God removes one of Adam's ribs and closes up the place with flesh. It's amazing to think of the God who had formed Adam out of dust now performing surgery on the man's body of flesh, blood, and bone.

It's important to remember several things about this action. This surgery was not to remove an imperfection from Adam's body, but to add something essential and beautiful to Adam's existence. It also speaks powerfully of the symbolic connection between man and woman. God could have created woman out of earth, as he did with man, but chose not to. The exchange of wedding rings, circumcision, communion, and other rituals are real-world actions with symbolic importance. In the same way, God's creation of woman from the rib of Adam meant to be seen as both literal and figurative.