Immersion can be described as the sense of a virtual reality experienced in a role-playing game. It includes such experiences as empathizing with a character, responding to narrative elements, and making decisions in-game rather than using meta-game thinking. For the player, immersion generally means geetting into their character and imagining something of their experience. For the gamemaster, immersion can be trickier to describe, but might involve imagining the world, reacting to the PCs as characters, and treating NPCs as entities with their own motivations and rationales for existence.
Immersion is roughly the antonym of meta-gaming. For this reason, immersion often represents an opposed viewpoint to theories based on goal achievement, such as GNS Theory. While someone engaged in immersion might experience a sense of the dramatic or a thrill at overcoming a challenge, the experience is immediate. Hence, an immersionist could share the same goal as a powergamer but have a markedly different play experience. While The Big Model partly came about to describe this type of involvement, ultimately this model breaks down because of the problem of simulation when applied to an immersive experience and the lack of description for what it means for someone to feel immersed. You can believe in a simulation and agree with its accuracy without being engaged.