This is a thought experiment designed by ibn Sina in order to prove God exists. You can read about ibn Sina's proof here.
Imagine a man created out of thin air. He's fully formed and fully grown, but he's suspended in the air from which he was created. However, this man's eyes are covered, and his ears are stuffed, his mouth closed. The air is still around him is still, so he can't feel anything or smell anything. This new man has no sensory input and has no acquired knowledge. So, ibn Sina raises the question: what's that falling man thinking about?
the falling man
Nowadays, when we think about this kind of scenario, we're reminded of Rene Descartes's famous "I think, therefore I am" ("cogito ergo sum"). The falling man couldn't be sure of anything in his surroundings. What he could be sure of is that he is thinking, therefore he must exist.
But ibn Sina delves into a slightly different question. As far as he's concerned, the falling man exists (for the sake of this experiment, at least), so he must be thinking. Ibn Sina is more concerned about what the falling man is thinking. Ibn Sina explores this through the process of elimination: he can't be remembering because he has no memories, he can't be reflecting on his environment because he has no way of knowing his environment, he can't be planning because... well... he has nothing to plan. So, ibn Sina posits that the falling man can only be reflecting on God. God, according to ibn Sina is the only "thing" the falling man would have knowledge of, seeing as he would have no terrestrial knowledge. Ibn Sina argues that knowledge of God is inherent in all living things (something rearticulated by almost all other Muslim philosophers) and that those thoughts exist in the mind before anything else. For ibn Sina, the falling man would be in a constant state of remembrance (dhikr) until some physical sensation interrupts it.
From a secular perspective, we can look at ibn Sina's thought experiment from the ground up. For most of us, our thoughts are made up of images or some sort of inner voice. For a falling man with no voice or no physical sensations to recall, they wouldn't be able to think in the same way that we do. Even the ways that we remember God are actions and words that we learn throughout our life, i.e. prayers. So, it's clear that this falling man is incapable of thinking in any way that we would consider thought. However, we can look at the works of Rumi or Khusraw who describe non-sentient matter (rocks, trees, etc) as in a constant state of remembrance of God. But this remembrance doesn't fit into our normal perception of what 'remembrance' is. It's not reciting prayers, performing actions, or repeating words. Instead, according to ibn Sina, it's simply our natural state without the distractions of the physical world. And when we practice meditation (bandgi), we try to push worldly concerns out of our minds to achieve that natural state again.