What is the "nature of reality" according to the Buddha? I have read that the Buddha said the cause of suffering is ignorance of the "nature of reality". Is this correct?

What is the "nature of reality" that the Buddha talks about?
Realities are called Paramattha Dhamma in Buddhism. There are 4 such realities.


Citta(consciousness)
ex: Vipaaka-citta, Karma, Kiriya-citta.
Cetasika(mental concomitants)
ex: Vedana, Sanna, Sankhara
Rupa(materiality)
ex: Patavi, Apo, Tejo, Vayo
Nibbana 



  What is the "nature of reality" that the Buddha talks about?


The first 3 realities listed above have 3 qualitative natures:


Anicca(impermanent)
Dukkha(unsatisfactory)
Anatta(impersonal)


The fourth reality only has the impersonal nature.

The cause for suffering is craving according to the 2nd noble truth. But craving cannot arise without ignorance. It also cannot be uprooted without eliminating ignorance. Ignorance is given as the first link of Paticca Samuppada, the dependent arising process which explains the origins of suffering.

I don't recall any sutta in which the Buddha making that exact statement, but it is inline with Buddhism. It would be more correct if you say "The cause of suffering is craving, which arises due to the ignorance of the nature of reality".