A long time ago, a famous Buddhist saint was doing a kind of meditation called “perfect wisdom”, and came to the realization that even the things that make us who we are (our form or matter, our sensations or feelings, our ability to sense things; our perceptions and conceptions, cognition, our picking and choosing; mental formations such as impulses or volition, mental habits, thoughts, ideas, opinions, prejudices, compulsions, decisions, and even consciousness itself) are empty and devoid of their own independent existence. And this realization completely relieved all of the saint’s suffering.
See, he found that even what we think of as solid form, what would be the opposite of emptiness, is exactly emptiness, and is nothing other than emptiness. This means that things like color, sight and sound, taste, smell and touch, and even the sensory organs we use to sense them (including the mind), aren’t what we’ve always assumed them to be. They are nothing, and at the same time they aren’t simply “nothing”. No adjectives can properly be used to describe these things, because they’re really only projections of our conceptions – and then, even those conceptions are empty.
In this way, we can see that even those teachings that we hold most dear are fabrications. When the Buddha taught that we can alleviate our own suffering through realization of the Four Noble Truths, even that teaching is a fabrication, and has no real self, no real form. Even the act of understanding, of attaining something, because there's really nothing to attain, nothing to understand.
The saint also realized that all enlightened beings throughout the ages had already found this to be true, and that their great realization was the foundation of their enlightenment. Thus, this realization equals enlightenment – but that enlightenment is also a fabrication, without its own permanent, independent form. To realize all this, then, is the basis for alleviating all suffering, and is a constant truth, not some made-up lie.
So the saint taught that when we meditate in Perfect Wisdom, we should think on this emptiness, and that we should let this emptiness become a kind of mantra, a kind of theme for our meditation.