What the old man tried to teach the anger some boy was - each time he looses his temper and hurt someone, even though he asks for forgiveness, the harm cannot be undone. This is just like the wooden board, even though the nails were removed, it is still full of holes that were caused by the nails. Even if it heals, it will take a very long time.
The wooden board signifies the victim of his anger, once harm is done, it would be forever scarred.
Therefore, we should check our behavior and control our temper not to hurt others.