Mindfulness meditation is a meditation practice which intentionally focuses upon the present moment, accepting and observing all sensations, thoughts, and emotions which may arise. One should not label, attach, wonder, think or force anything, but instead should just witness and simply be (though it can be much more difficult than one would imagine).
When practicing a Mindfulness meditation, one may sit or lie down. Close attention can be payed to the movement of the breath. When one breathes in, they can become aware that they are breathing in, and recognise how it feels. When one breathes out, they can also become aware that they are breathing out and so on. Throughout the meditation, one can direct attention towards the breath or pay attention to the sensations, thoughts and feelings that arise from an observational stand point. Becoming the witness and being aware of all that is currently happening but not attaching to it (Internally or externally). Thoughts can be seen as ships passing before you on the ocean, and as the observer or pure presence, you are merely observing them from the shore line. Not engaging with them, nor labelling them, nor being carried off with them, just simply recognising their existence and allowing them to be and run their course.