They thoroughly understand the nature of all dharmas: everything is empty and without self.
The word “dharmas” refers to phenomena. The word “nature” refers to innate character or noumenon. The noumenon of all phenomena is empty and quiescent. When the Buddha said that all phenomena are empty, he meant that the noumenon is empty: phenomena do not have self-nature and are empty in themselves. Everything is empty and without self. “Self” implies being in control. “Everything is empty and without self” means that no one controls the phenomena. Then, how do phenomena come about? They arise from the combination of various conditions. Boundless conditions gather and generate them. Therefore, phenomena do not have self-nature or self-identity.
When we are clear about this truth, we should absolutely not attach to any phenomena or give rise to any thought. The mind should always be pure, impartial, and awakened.