Mindfulness
Traditionally, mindfulness is not about "stress reduction" or "being in the moment." But for (non-Buddhist) Westerners, stress-reduction and being in the moment are often desirable byproducts of this practice: it provides a real experience of the fleeting, insubstantial nature of feelings you've learned to fear. (The same goes for feelings you crave, or chase after.) It allows you to tolerate difficult states of mind as you observe them. It can clear the way for skillful action in difficult situations.
I am completely nondenominational in my work. But, since mindfulness comes from Eastern traditions, it's good to remember how it's used there. In its Buddhist meditational matrix, mindfulness helps meditators observe and 'unpack' moment to moment awareness without getting caught up in mental / emotional content. It enables them to observe how they do get caught up, and to detach. The goal is to enable practitioners to see through and work –actively and safely – with personal suffering and "delusion" (in its Buddhist definitions).
Buddhist meditators are seeking liberation from suffering – perhaps a more radical way than we will be approaching in therapy per se. But to put “mindfulness” in a Western psychotherapy frame: we can use mindfulness to work with the things that keep us stuck, to clear away whatever keeps us from seeing our situation, and our world, clearly. When we see clearly, we can act wisely and skillfully.