Short answers, pulled from the story.
The Laṅkāvatāra Sūtra is a Mahayana Buddhist sūtra recounting a teaching between Gautama Buddha and the bodhisattva Mahāmati, set on the mythical island of Laṅkā, ruled by Rāvaṇa, king of the rākṣasas. It covers Yogācāra mind-only philosophy, the storehouse consciousness, buddha-nature, emptiness, and vegetarianism.
It is generally believed the sutra was compiled between 350 and 400 CE, according to Asanga Tilakaratne, though many scholars think its introductory chapter and final two chapters were added later. Gishin Tokiwa has separately proposed compilation in Sri Lanka between 411 and 435.
The sutra is notably important in Chan and Zen Buddhism because it discusses sudden enlightenment, later called dun in Chinese and debated across Chan, Zen, and Seon traditions. Tradition also holds that the Guṇabhadra Chinese translation passed from Bodhidharma, the founder of Chan Buddhism, to his successor Dazu Huike.
The Laṅkāvatāra Sūtra teaches that all phenomena are manifestations of the mind itself, a philosophy tied to the doctrine of svacittamātra. It states that what is seen as something external is nothing but one's own mind, expressed as svacitta-drsya-mātram.
The Five Dharmas are Nimitta, images and appearances, Nāma, naming, Samkalpa or Vikalpa, discrimination, Samyagjñāna, right knowledge, and Tathatā, Suchness or ultimate reality. According to Saganuma, this schema is essential for understanding the sutra's basic worldview.
English translations include Thomas Cleary's 2012 translation from the Sanskrit edition, Gishin Tokiwa's 2003 translation based on Guṇabhadra's Chinese text and a restored Sanskrit, Red Pine's 2012 composite translation, and D.T. Suzuki's translation, first published in 1932.