What did he mean by 'I don't understand the plot of' The Secret that Can't Be Revered'?

" The Unspoken Secret " was an animated film adapted from the song of the same name sung by Jay Chou. It told the story of the unspoken secret between a little boy and a girl. The story took place in a small town called "Xingyue County". The protagonist, Xiaoyu, was a student here. He became friends with a boy named Ye Xianglun. Ye Xianglun was a musical genius who was good at playing the piano, and Xiaoyu was his student and fan. By chance, Xiaoyu and Ye Xianglun discovered a mysterious notebook with some knowledge about time and space and forgetting. They decided to explore the secrets of this mysterious notebook together and try to unravel the secrets behind it. In this process, they experienced many difficulties and dangers, including the passage of time and the intersection of time and space. In the end, they discovered a shocking secret: Xiaoyu's parents had met in this small town at some point in time and left a key message that they had forgotten. Ye Xianglun used his musical talent to help Xiaoyu and others find this information and use it to solve many problems. In the end, they successfully uncovered the secret of the mysterious notebook and returned to their own era. The difficulty of the story was that the passage of time and the intersection of time and space required the audience to have a certain understanding of the concepts of time and space to understand.