It is actually a type of perception. An altered perception if you wish to call it that (in worldly terms), though the Buddha would hold that what is experienced after conditioned experience drops away is actually the true perception… what we commonly take as normal is the delusion. Further developing these types of supramundane perception is the way to Nibbana.
And what is the way of developing immersion further that leads to mindfulness and awareness? It’s when a mendicant knows feelings as they arise, as they remain, and as they go away. They know perceptions as they arise, as they remain, and as they go away. They know thoughts as they arise, as they remain, and as they go away. This is the way of developing immersion further that leads to mindfulness and awareness.
There is a way to intellectually understand this unbounded experience as follows…
Put aside the Consciousness aggregate for a moment and consider the Form aggregate.
Would you say that your Body (Form) is distinct from other living beings?
The instinctive answer is “Of course”… but that is a delusion.
The body (Form) is sustained by Food. The apple (Form) you put into your mouth, at some stage will become you. Your body too is not constant, it is forever in change. Consider… your skin, hair, nails are forever growing, being shed off. At what stage does the hair of your head stop being your hair and become other? And what happens when that self same hair is swept up, admixed with the hair of others (whose Form is similarly in movement), deposited at the root of the backyard apple tree… to make new apples next year… which again become you!
So when it comes to Form, the entire Rupa Khanda of the World is actually one… the only boundaries we place are artificial constructs of our mind. You really aren’t as separate from that apple as you may think!
The Rupa of the World is constantly flowing around and through … like the water of a river flows around and through a Whirlpool. Your body has just about as much separation/ reality as that Whirlpool.
This is a way in which Form can be understood to be Not-Self. (You may have noticed that I have used a modified version of Satipatthana for this exercise!)
You may separately consider the other aggregates too, though the exercise gets progressively trickier …
Meditation in terms of Jhanas offers the opportunity to viscerally know the same thing as an experienced fact. Hence it offers a window into the true nature of Reality. Actual lived Experience is held in far greater stead over dry intellectual understanding when it comes to its capacity to free the Mind (I am reminded of Ajahn Brahm’s joke of the gourmet who ate the menu instead of the meal ). Hence its importance.