That is to say "no layers" or "right on the substrate".
Just a guess though.
Ok, I feel very pedantic saying this, but the substrate is the thing that is mostly semi conductors :P.
But don't take this so seriously, I work in semiconductor manufacturing and even I don't take it that seriously.
All possible abstraction removed = bare metal
On x86/x64, the chips have long migrated to an internal RISC like architecture, with an Assembly => micro-op translation step on the decoding unit.
Yes, assembly is an (mostly fairly thin, on non-microcoded hardware) abstraction over machine code.