Mindfulness is the sensitive one. He notices things. Concentration provides the power. He keeps the attention pinned down to one item. Ideally, mindfulness is in this relationship.
Mindfulness picks the objects of attention, and notices when the attention has gone astray.
Concentration does the actual work of holding the attention steady on that chosen object. If either of these partners is weak, your meditation goes astray. Concentration could be defined as that faculty of the mind which focuses single mindedly on one object without interruption. It must be emphasized that true concentration is a wholesome one-pointedness of mind. That is, the state is free from greed, hatred and delusion.