It's a bit like a mesh beekeeper's mask, though made of disposable fabric. It's supposed to dissuade a prisoner from spitting at guards by drastically reducing the pssibility that any spit gets through the screen to their target.
I think there might have been a problem with prisoner spitting, partly because with hands cuffed and so on there weren't many ways to lash out at the guards.
"'Multiple incidents within the juvenile detention facilities have revealed that the NT Government prosecutes policies against Aboriginal children which include spit-hooding, gassing, hand cuffing, shackling and extensive periods of unlawful solitary confinement. Treatment such as this you wouldn’t think possible in any civilised nation. Let there be no doubts as to whom we are talking about here. This is all about Aboriginal children.'