Short answers, pulled from the story.
A dharani is a lengthier Buddhist mantra, incantation, or recitation, almost always composed in Sanskrit. Dharanis function as mnemonic codes, protective charms, amulets, and encoded summaries of Buddhist doctrine, and they appear in the texts of all major Buddhist traditions.
The word dhāraṇī derives from the Sanskrit root meaning "to hold or maintain." The same root gives the word dharma. In early Buddhist usage, the term referred specifically to mnemonic devices used to retain elements of Buddhist doctrine in memory.
The 8th-century dharani texts from East Asia are described by scholars including Robert Sewell as the oldest authenticated printed texts in the world. The early-eighth-century dharanis discovered at Bulguksa temple in Gyeongju, Korea, are considered the oldest known printed texts. The demand for printed dharanis among Buddhist lay devotees is credited with stimulating the development of woodblock printing technology.
Empress Shotoku of Japan ordered one million dharanis produced and distributed through Buddhist temples after an attempted coup against her court. Each dharani was a printed prayer or charm in Sanskrit on a thick paper strip, sealed inside a miniature hollow wooden pagoda. These are among the oldest known printed texts in the world, produced between 764 and 770 CE.
Dharani chanting was one of the common methods of healing during the Nara period in Japan. A ritsuryo code dated 718 CE explicitly permitted dharani chanting for healing the sick in accordance with Buddhist dharma, while forbidding unauthorized medical, military, or political uses. Monks described as healer-meditation masters are mentioned in a document from 797 CE chanting dharanis to protect the ruler's family.
Eugène Burnouf noted that dharanis and mantras are very similar, with dharanis being much longer. According to the writer Red Pine, dhāraṇī came to refer to meaningful, intelligible phrases, while mantra referred to syllabic formulae not meant to be understood. The Japanese monk Kōbō Daishi classified mantras as a special class of dharanis and argued that dharanis appear in both esoteric and exoteric Buddhist ritual, while mantras are restricted to esoteric practice.