The Heart Sutra runs to a couple of hundred syllables and ends by declaring itself a mantra, the great mantra, the unequalled mantra, and then giving it. The words are forms of a verb of going: gone, gone, gone beyond, gone altogether beyond, awakening, and a closing exclamation.
The syllables are feminine in form, and the natural addressee is Prajnaparamita, the perfection of wisdom personified as a goddess, who is the subject of the whole literature the sutra compresses. Read that way it is a vocative address, not a description of a journey, though the journey reading is the one almost every commentary and translation offers.
The sutra itself is the most recited text in East Asian Buddhism, chanted daily in Chinese, Japanese, Korean and Vietnamese monasteries. Its content is a systematic negation: it takes the standard Buddhist analytical categories, the five aggregates, the sense bases, the twelve links, the four noble truths, and denies each of them in turn, ending with no attainment and nothing to attain. That the denial extends to the four noble truths is what makes the text startling on first reading, and the Madhyamaka reading is that what is denied is inherent existence, not the truths.
Dharani is the older term for a formula of this kind, distinguished in Indian usage from mantra by its function as a mnemonic condensation of a teaching rather than an invocation. The closing formula of this sutra is dharani-shaped: it compresses the text it ends.