Śūnyatā or emptiness is a Buddhist technical term which refers to the fact that phenomena lack permanence and substantiality (or essence). This mantra is used in tantric rituals to remind the yogin of this basic Buddhist teaching, and to try to evoke the experience of it - normally we experience a series of jolts when we discover that things don't last, or provide satisfaction because we fail to see the true nature of things. The Śūnyatā mantra helps us tune into impermanence and insubstantiality. The mantra appears in visualisation practices as a prelude to visualising the yidam.
There is another mantra which is commonly called the Śūnyatā Mantra which I have labelled the Purity Mantra to distinguish it from this mantra.
oṃ śū nya tā jñā na va jra sva bhā vā tma ko 'haṃ
oṃ śūnyatā jñāna vajra svabhāvātmako 'haṃ
Translates literally as: "oṃ emptiness knowing diamond self-nature-essence I"
In The Cult of Tārā" Stephan Beyer renders it: Oṃ. I am the very self whose essence is the diamond knowledge of emptiness.
Or it could go: Oṃ I am the vajra essential-own-being which is the knowledge of śūnyatāSvabhavatmako = svabhāva + atma + (suffix) ko. Probably something like "the one with own-being-self". Funny to see ātma used in a positive sense in a Buddhist context huh?