In this verse, the Buddha says that the greatest corruption is ignorance. Ignorance is an unknowing; it is not knowing something. What is it that we do not know that is our greatest corruption because it is the underlying cause of all our suffering and confusion? It is ignorance of the way things actually exist as opposed to the way they appear. It is an unknowing of reality.
The mistaken way we are viewing everything is that we believe that all things exist exactly as they appear, in an independent and self contained way. We believe things exist independently of our perception, that a cup is a cup independent of our labeling it a cup. In fact, all things are dependent arising; they depend on many factors bringing them into existence including our own perception and labeling of them. We label ourselves good, bad, tall, short, skinny, fat. We label our life good or bad and all of our experiences we label as good or bad. We do not label them as appearances to our mind. But, in reality ourselves, other people, and all the experiences of our life are actually appearances that we have created with our mind. The special wisdom is called the wisdom of emptiness. This is a wisdom that realizes that our reality and all the things that we see are empty of inherent existence. Things do not exist inherently, independent of causes and conditions or the perception of our mind. Things do exist, but they do not exist the way that they appear.
More corrupt than these,
Is ignorance, the greatest corruption.
Having abandoned this corruption,
Monks, remain corruption free! (Verse 243)
--Buddha, the Dhammapada
Buddha. The Dhammapada, translated by Gil Fronsdale. (2011). Shambala, pp.63.
Guide to the Bodhisattva’s Way of Life by Acharya Shantideva. Translated into
Je Tsongkhapa. Great Treatise on the Stages of the Path to Enlightenment, Volume 3. Translated by the Lamrim Chenmo Translation Committee. Joshua Cutler, Editor-in-Chief, and Guy Newlan, Editor. Pages 1961, 2014, 2019.