Here's some background
Although even as a young man, Tsong Khapa's learning was unexcelled and his fame as a teacher and meditation master widespread, he remained dissatisfied with his realisation of the ultimate view of reliaty, the wisdom of emptiness.
In particular, he felt there were subtle points relating to the profound madhyamika philosophy as propounded by the Indian masters Nagarjuna, Buddhapalita and Chandrakirti about which he still needed clarification.
Therefore, at the age of forty he withdrew from active teaching and put himself into intensive retreat on these very points. Then, as related in a poem recounting his mystic experiences, written by one of his disciples and addressed to Je Rinpoche himself:
One night you dreamed of Nagarjuna
And his five spiritual sons
Amongst themselves discussing
The fabric of dependent origination.
From their midst came Buddhapalita
Who touched you with a scripture. Tsong Khapa awoke with a feeling of great bliss and immediately opened his copy of Buddhapalita's text. As recounted in his biography:
While he was reading the words,
"the self is not the same as the [mental and physical] aggregates, nor is it anything other than the aggrefates" he effortlessly experienced the ultimate realisation of absolute reality, along with perfect understanding of all the ... subtleties concerning the authentic view.
Overfloweing with joy and faith in Shakyamuni Buddha, the original source of this enlightening realisation, he wrote the poem entitled "In Praise of Dependent Arising."
From: Images of Enlightenment: Tibetan Art in Practice, By Jonathan Landaw, Andy Weber