One of my unforgettable memories of my school in Xinjiang is that of the lunches we brought from our homes.
What is the meaning of that of the lunches?
“that” is a pronoun which stands for “memory” (referring back to the earlier part of the sentence). So, in full,
“One of my unforgettable memories of my school in Xinjiang is the memory of the lunches we brought from our homes.”
This sounds a bit repetitious, so the author has substituted “that”.
Thank you very much.