I'll give the appropriate (and hopefully rigorous enough) context for the explanation:

A Chair exists in three seemingly contradictory ways, yet it is because of this contradiction it exists at all.

A "chair" is at the first level but a mere concept and definition which denotes a class of objects. This is the first Nature of a chair, of pure mental construction. "Chair", then, only has any meaning at all because the symbols which signify the concept (c-h-a-i-r) exist within a consistent, accepted framework, i.e. the English language, and its function therein. How else would you know, unthinkingly, what I am referring to when I write 'chair'?

The imputation "chair" can only exist, however, in tandem with that which it is agreed to characterize. This is the second nature - the relative nature. A chair exists as but a set of causes and conditions underlying the imputation "chair" - the materials of which it is made and the associated processes of its construction.

"Chair" has no meaning at all without that which it characterizes.

As the Dalai Lama elegantly says: "Phenomena are merely imputed by terms and conceptuality in dependence upon their basis of imputation."

The real nature of the chair is, therefore, that it is nonexistent, yet it is precisely because it is nonexistent that it appears. Another way to say this, is to consider that the materials of which the chair is made - that is, its relative, conditional existence - only exist themselves through the imputation "chair". Without the imputation "chair", these particular causes and conditions would be undistinguished. Ultimately, the processes comprising the chair are themselves empty, from their own side.

This is truly profound when we recognize conceptual markers like "chair" as an attachment to the form of these empty conditions. Staggeringly, it is because of (illusionary) mental construction that anything exists at all, which is also to say that there are no phenomena at all.

Ponder: what would it, therefore, mean to detach from form, without obliterating form?

The following is Fa-tsang's explanation of the true profundity of this doctrine. The first paragraph is a bit of context I have included from the author and compiler, Thomas Cleary, and the entire excerpt is taken from his excellent work: Entry into the Inconceivable: An Introduction to Huayan Buddhism.

[The] three natures may be easily understood in terms of the relationship of sense faculties, consciousness, and sense data. The nature of mental construction is the conceptual order (including perceptual order) which we learn to accept as "the world." The relative nature is the conditional existence of the world of experience as a relationship of sense faculties, sense consciousnesses, and sense data. Thus we can see that the order is a mental projection or construction rather than an inherently fixed property of an objective world as conceived. The real nature is the lack of independent existence of senses, consciousnesses, and data-existing only in relation to each other, as such they have no absolute existence.

In his "Treatise on the Five Teachings," Fa-tsang explains that each of the three natures has two senses. The true or real nature has the sense of not changing and the sense of going along with conditions: the sense of not changing refers to the emptiness or nonabso­luteness of that which is conditional; the sense of going along with conditions refers to conditional existence, outside of which there is no emptiness. The relative nature has the sense of seeming to exist and the sense of having no essential nature-that is, conditions have apparent existence, relative to one another, but they have no abso­lute existence independent of each other. The nature of conceptual clinging has the sense of mentally existing and the sense of really not existing-that is, definitions exist in the mind but not in objective reality.

Fa-tsang explains that in terms of the true nature's sense of not changing, the relative nature's sense of having no essential nature, and the conceptual nature's sense of not really existing, the three natures are identical. It is in this sense, he points out, that scripture says, "Sentient beings are of themselves nirvanic and are not further extinguished." Fa-tsang goes on to say that in terms of the true nature's sense of going along with conditions, the relative nature's sense of seeming to exist, and the conceptual nature's sense of mentally existing, the three natures are not different; it is in this sense, he says, that the Hua-yen scripture states, "The reality-body revolving in mundane existence is called sentient beings."

u/En_lighten, u/UsYntax