Mindfulness does not aim at anything. It just sees whatever is already there. Mindfulness is a broader and larger function than concentration. Concentration is exclusive. It settles down on one item and ignores everything else.
Mindfulness is inclusive.
It stands back from the focus of attention and watches with a broad focus, quick to notice any change that occurs. If you have focused the mind on a stone, concentration will see only the stone. Mindfulness stands back from this process, aware of the stone, aware of the concentration focusing on the stone, aware of the intensity of that focus and instantly aware of the shift of attention when concentration is distracted. It is mindfulness which notices the distraction which has occurred, and it is mindfulness which redirects the attention to the stone. Mindfulness is more difficult to cultivate than concentration because it is a deeper-reaching function.