This is the tale that the Clerk tells the pilgrims on the road to Canterbury, but he wants to make it very clear that it doesn't originate with him. "I wolde yow telle a tale, which that I / Lerned at Padwe of a worthy clerk" (26-27).
That "worthy clerk," we immediately learn, is Francesco Petrarch (or Petrarca), an important Italian poet of Chaucer's day. In saying that Petrarch "taught" him the tale, the Clerk probably just means that he read it in a book of Petrarch's works. It was common for medieval authors to refer to their encounters with another author's work as if they had actually heard it in person from the author; this reflected the medieval view of writing as a means of hearing the "voices" of those who were not present.
But why does the Clerk go to such trouble to acknowledge the original source of his tale? This kind of borrowing was totally accepted; it was even an expected means of becoming an author, so it's not like the Clerk needs to fear an accusation of plagiarism.
The most likely explanation is that the clerk wants to advertise his familiarity with Petrarch as a means of demonstrating his learning. By emphasizing how his own tale is also Petrarch's tale, the Clerk demonstrates his familiarity with learned culture and makes himself a part of it by riffing on it.