What does Mark 6:52 mean?

There are several things the Twelve do not understand about the loaves, or about anything that Jesus does. "Understand" is from the Greek root word suniemi and means to analyze the evidence and come to a conclusion. Their confirmation bias tells them that Jesus is the Messiah, come to make their lives easier and richer. What they don't see is that Jesus isn't there to make them prosperous but to provide what they need, which applies only to what's required in order to accomplish God's work (Philippians 4:12–13).

"Hardened" comes from the Greek root word poroo. It brings to mind an elephant's foot, heavy layers of tough skin that prevent any dexterity or feeling. At the same time the Twelve's spiritual understanding is blinded by their hard hearts, they worship Jesus as the Son of God (Matthew 14:33). It is possible to talk the talk without understanding the words. We see this in churches that have gone astray in their understanding of God's expectations as well as cults who use familiar words but have heretical teaching. Words are useless if they are twisted away from the truth.

This entire episode is an illustration of God giving the Twelve what they need. Although they didn't get to debrief their missions trip, they do get time away from the crowds. They didn't bring food, but God provided (Mark 6:39–43). The next day, when the crowd finds them again, Jesus will say, "I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst" (John 6:35). Jesus will show that man lives on the Word of God, not by bread alone (Matthew 4:1–4) when He tells the Twelve that to do God's will is more satisfying than bread (John 4:31–34). For a while yet, however, the Twelve will miss the point that if we trust and obey God, He will take care of us (Mark 8:16–21).