What does John 6:12 mean?

The initial problem faced by the disciples seemed insurmountable: a crowd of thousands, looking for food. The throng wasn't going to leave (Mark 6:36), there was not enough money to feed everyone (John 6:7), and all they could scrounge up was barely enough to feed one boy (John 6:9). In response, Jesus miraculously provides not just a solution, but an abundance, more than was needed to solve the problem (John 6:13). Everyone eats "their fill," from the Greek eneplēsthēsan, which refers to being satisfied, full, or satiated. Every person in that massive audience was able to eat until their hunger was gone.

This is meant to teach two primary lessons. First, the disciples were overly concerned with their own efforts. Ignoring the problem, worrying about money, and despairing over limited resources were all signs that their first thoughts were earthly, not heavenly. Instead of lamenting how massive the need is, our approach should be to simply bring God all we have, and let Him handle the results. When God gives us a challenge, He expects us to apply effort and resources to it (Colossians 3:23–25), but at the same time He wants us to proceed in faith, not in despair (Luke 10:2). That begins by putting the matter in God's hands, first, which is exactly what Jesus did.

The second lesson is that of perspective. The disciples' money seemed impossibly limited; the boy's lunch seemed impossibly small. And yet, in the hands of Christ, what seemed small and insignificant became so abundant that people had to be careful not to waste the leftovers! In the same way, while our personal skills or our resources might seem pathetic, Christ can leverage those tiny assets into powerful results. The power is His, and the results are His. We are only called to faithfully give Him all we have, even if it seems like it's not enough.