What is a temporal loop in Chapter 6?

From the text:

We create temporal loops in which peculiar folk can live indefinitely.”

“A loop,” I repeated, remembering my grandfather’s command: find the bird, in the loop. “Is that what this place is?”

“Yes. Though you may better know it as the third of September, 1940.”

I leaned toward her over the little desk. “What do you mean? It’s only the one day? It repeats?”

“Over and over, though our experience of it is continuous. Otherwise we would have no memory of the last, oh, seventy years that we’ve resided here.”