Matter. Material monism is by definition referring to matter.
I am not sure what the confusion is here.
Every instance of Meaning must have something “that means”.
How is meaning related to ontology? Monism is a about what exists, not what something means. Semantics is the domain for this.
As I have said before, we ask what exists (ontology), how do we know (epistemology) and what is its meaning (semiotics). Together then we ask what is the meaning (semantics).