saluete amici!
I think the reference to the Virgin Mary is meant to signify that while the biblical accounts of course narrate her conception and bearing of the baby Jesus, her chastity and moral/sexual purity remained unsullied and intact.
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Edited afterthought: after consulting a theologian-friend (herself a protestant), I have now satisfied myself that this notion of the Burning Bush story being a presage of the status of the Virgin Mary must be due to some eccentric mediaeval papal whimsy, for certainly it has no obvious or unambiguous biblical justification. But it does at least seem clear that ignis ardens, non comburens mean that the bush, while 'ablaze', was (in the words of the King James Bible) 'not consumed'.