It is just looking. Therefore, desire and aversion are not involved. Competition and struggle for achievement have no place in the process. 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.