The Two Truth is on the short list of most important Buddhist concepts. This is material to chew on for a lifetime of practice, not something we get on a first or even second or third read through.
The Two Truths
Excerpted from the Stanford Encyclopedia of Philosophy
The theory of the two truths is the heart of the Buddha's philosophy. It serves as the mirror reflecting the core message of the Buddha's teachings and the massive philosophical literature it inspired. At the heart of the theory of the two truths is the Buddha's ever poignant existential (based on experience) and soteriological (salvation) concerns about the reality of things and of life. Nirvaṇa, ultimate freedom from the suffering conditioned by desires, is only ever achieved, according to the theory of the two truths, from a correct understanding of two truths. Knowledge of the conventional truth informs us how things are conventionally, and thus grounds our epistemic (the nature and scope of knowledge) practice in its proper linguistic and conceptual framework.
Knowledge of the ultimate truth informs us of how things really are ultimately, and so takes our minds beyond the bounds of conceptual and linguistic conventions.
In India the theory of the two truths the Buddha had explained, of course without much elaboration, stimulated rich philosophical exchanges amongst the Buddhist philosophers and practitioners. The transformation of the two truths theory from a simple to a complex system of thought with highly sophisticated concepts is perhaps the most significant contribution resulting from the schisms the Buddhism experienced after the Buddha passed away (ca. 380 BCE). Various schools with varying interpretations of the Buddha's words soon appeared in Buddhism, which resulted in rich and vibrant philosophical and hermeutic (the way we study written texts) atmosphere.
After the Buddha the philosopher who broke new ground on the theory of the two truths in the Madhyamaka system is a South Indian monk, Nāgārjuna (ca. 100 BCE–100 CE). Nāgārjuna saw himself as propagating the dharma taught by the Buddha, which he says is precisely based on the theory of the two truths: a truth of mundane conventions and a truth of the ultimate.
He saw the theory of the two truths as constituting the Buddha's core teaching and his philosophy. Nāgārjuna maintains that those who do not understand the distinction between these two truths would fail to understand the Buddha's teaching. This is so, for Nāgārjuna, because (1) without relying on the conventional truth, the meaning of the ultimate cannot be explained, and (2) without understanding the meaning of the ultimate, nirvāṇa is not achieved.
Nāgārjuna's theory of the two truths is fundamentally different from all theories of truth in other Indian philosophies. Hindu philosophers and other Buddhist sects all advocate a foundationalism of some kind according to which ultimate reality is taken to be the “substantive reality” or foundation upon which stands the entire edifice of the conventional ontological structures where the ultimate reality is posited as immutable, fixed, irreducible and independent of any interpretative conventions. That is so, even though the conventional structure that stands upon it constantly changes and transforms. [This, of course, contradicts the most fundamental teaching of the Buddha, that all things are impermanent, and is, obviously, illogical, as Nagarjuna repeatedly explains in his writings.]
Nāgārjuna's central argument to support his radical non-foundationalist theory of the two truths draws upon an understanding of conventional truth as tied to dependently arisen phenomena, and ultimate truth as tied to emptiness of the intrinsic nature.
Since the former and the latter are coconstitutive of each other, in that each entails the other, ultimate reality is tied to being conventionally real. Nāgārjuna advances important arguments justifying the correlation between the conventional truth vis-à-vis dependent arising, and emptiness vis-à-vis ultimate truth. These arguments bring home their epistemological and ontological correlations.
He argues that wherever applies emptiness as the ultimate truth, there applies the causal efficacy of the conventional truth and wherever emptiness does not apply as the ultimate truth, there does not apply the causal efficacy of the conventional truth.
According to Nāgārjuna, ultimate truth's being empty of any intrinsic reality affords conventional truth its causal efficacy since being ultimately empty is identical to being causally produced, conventionally. This must be so since, for Nāgārjuna, “there is no thing that is not dependently arisen; therefore, there is no such thing that is not empty.
Why is this so important? Because, only if we get it that things really aren’t what they appear to be, can we stop our grasping and clinging and attaching and end our suffering.