The A button represents the “courageous” action, which in the case of a battle would be attack. The B button represents the opposite – in this case, evasion. You don’t get damaged, but the number of tears you get after you defeat the enemy, shown in a cyan bar at the top of the screen, decreases.
When I say “in the case of a battle,” it means that these controls are reflected outside of battle. For example, you can do the “right” action with A when someone requests you to do something, causing you to actually do the action, or you can press B and kick the person.
Simple, yet effective. That seems to be the theme Marvelous was aiming for: a game that reflects the way a child sees the world.