How does "that" work in this sentence by Ken Follett?

At the beginning of Ken Follett’s book The Pillars of the Earth, he wrote:

"It is in this rich tapestry, where kings and queens are corrupt, that the common man shows eternal promise — and one majestic creation will bond them forever…"

It looks like "that" means "when" or "while", but I found nothing in dictionaries.

I did find an explanation in Merriam-Webster saying "used as a function word to introduce a subordinate clause modifying an adverb or adverbial expression <will go anywhere that he is invited>", but in the sentence "that" is not modifying an adverb but "tapestry".