I don’t usually use the word mindfulness. Mainly because I prefer the word noticing, and also because I often am more interested in other aspects of the exploration.
The way I understand mindfulness is noticing.
A noticing of content of experience, which can be differentiated into sensory experiences and imagination (mental representations of sensory experiences, words).
A noticing of how sensory experiences (and especially sensations) and imaginations come together to create experiences that seem real, solid, and substantial. A noticing how how this sense of reality dissolves when we do the forms of noticing described here.
A noticing of space outside and inside of this content of experience.
A noticing of all as awareness.
A noticing of the emptiness all of that happens within and as.
And so on.
There is another way of understanding mindfulness which I like. Mind-full = noticing the mind “full” of presence. The whole field of experience is already and always presence. (Or awareness, or awake space, or emptiness allowing all of this.) And as we notice this, the “center of gravity” can shift from being caught in content of experience to this “context” of all experience. We may even notice this presence as what we are. Content of experience is presence. (And there is an emptiness which all this happens within and as, which all also and more fundamentally is.)