It defines a proponent of meat-eating and human consumption of animals in general. It'd be what meat-eaters would call themselves if they could only accept that their diet stems from a philosophy of speciecism and a belief that humans are superior to other sentient beings.
Given that meat eating it is a choice and not an actual necessity in civilised societies, carnism is a philosophy that seeks to defend carnists in their abject subjugation and oppression of non-human animals by justifying their decision and making animals appear as mere objects.
John: Fuck, I love me some bacon.
Mary: Please don't be such a carnist.
John: But I like BACON. What's a carnist anyway?
Mary: Someone who thinks that animals are meant to be eaten and feels no remorse for it.
John: But animals have to be eaten.
Mary: My point exactly.