I heard there was a pope in the thirteenth century who resigned. Is this true?

Yes. Pope St. Celestine V reigned for about the last five months of 1294. He resigned with these words:

We, Celestine V, Pope, moved by legitimate reasons . . . do freely and voluntarily resign the Pontificate, the place, the dignity, occupation and honors which we expressly renounce, and we give full and free faculty to the College of Cardinals canonically to elect a Pastor of the Universal Church.

Celestine had been elected pope after a papal interregnum of two years, during which time there had been fierce fighting between rival factions within the cardinalate. A Benedictine monk and hermit almost 80 years old and unaccustomed to ecclesiastical politics, Celestine found the office too much to handle and resigned. He was succeeded by Boniface VIII.

Celestine died shortly after his resignation and was canonized within 20 years of his death.