Alright, so here's the definition for those who don't want to bother Googling it:
"an underlying layer or substance, in particular a layer of rock or soil beneath the surface of the ground; a foundation or basis of something."
This may be symbolic: we're starting to get past the first layer of the ogre of mystery and into the "substrata," or the part where we actually get some substance. There's gonna be less smoke and mirrors from here on out.
I doubt that "substratum" refers to the actual image because, as @PakariNation99 noted, Faber clarified that that little GSR-looking dude isn't underground. What's above him is probably ice, likely meaning he's on an ice planet of some kind, or at least under an ice cap. This would fit if Faber wants to incorporate environmentalism into 3IONICLE, with this planet/biome corresponding to our real-world ice caps, threatened by climate change.
The little dude there also seems to be alone in the cave, so if it's a re-imagining of Ko-Wahi that he's in, he may not find civilization until he takes a very long walk, if there's anybody down there at all. In fact, this whole region may be...
...and we might see similarly drastic changes to the other elemental zones, if there are any.