This is the mantra from the Prajñāpāramita Hridaya Sutra or Heart Sutra. Prajñāpāramita the bodhisattva has another mantra. To see the whole Heart Sutra in siddham script look here

Mantra

Transliteration

ga te ga te pā ra ga te pā ra saṃ ga te bo dhi svā hā

Notes on the Heart Sutra Mantra

Conze's translation reads: "gone, gone, gone beyond, gone altogether beyond, oh what an awakening, All hail!"

The words of the mantra are in the feminine vocative case. The vocative case indicates that someone is being addressed. so that gate means not simply 'gone', as it is usually translated, but "Oh she who is/has gone!".

The Heart Sutra itself describes the mantra as "Mahāmantro, mahā-vidyaa mantro, ‘nuttara mantro samasama-mantra.h" which Conze translates as "The great mantra, the mantra of great knowledge, the utmost mantra, the unequalled mantra, the allayer of all suffering". These are traditional epithets for the Buddha, so the mantra is being likened to, or equated with, the Buddha.

Although there are many commentaries on the Heart Sutra, none is really able to satisfactorily account for the mantra and it's relationship to the text. Many commentators gloss over the mantra, and some even try to ignore it altogether. Only Conze in his Buddhist Wisdom Books makes a credible attempt at explaining the mantra.