The question can be answered at a prosaic neuroscientific level that skirts the question of consciousness (i.e. attention as an executive function in the neocortex), and at a philosophy of mind level. Both are kinda useful.

I would recommend the following to get a basic grasp of the subject:

The Society of Mind by Marvin Minsky
Consciousness Explained by Daniel Dennet
How the Mind Works by Steven Pinker

if you are in the mood for some heavier lifting, try The Conscious Mind by David Chalmers.

I wouldn't recommend books that are overly based on neuroscience. Even the best ones sometimes make basic metaphysical framing errors like unwittingly describing phenomena in terms of the 'Cartesian Theater' idea, which is deeply flawed. Brain Rules by John Media is among the better ones, that doesn't make any serious philosophical mistakes or wander overreach from biology to psychology/philosophy.

There is almost no overlap or crosstalk between this philosophy of mind/cognitive science/neuroscience tradition and mystic traditions. Each assumes the other tradition is hopelessly deluded.

Though Mark Bruns has tried, I think the best way to describe the mystic answer to "what is attention" is that there is no answer. All you get is practices to help you control it, without intellectualizing the topic at all. Or less politely, the mystic answer is "shut the hell up and breathe."