The position parameter mimics the placement of a listener in a closed space, from being up close to the sound source (less reverb) to far from it (more reverb).
Think about a hall or a chamber with a stage. Position would set you close to the stage or far away from it, making the reverb a lot stronger in the rear of the room. It’s a raw “front/rear” placement of the listener which works quite nicely. In a more modern approach to spatial perception/surround sound you’d also need to change EQ and sum part of the stereo front sound to mono to throw it in the rear space, which is what more poweful algorithms do in higher end processors, but still.. this simple position parameter does a good job at simulating space by mixing the diffused early reflections over the reverb in the front, while doing the opposite in the rear.
How does this affects the sound? You should answer the question, using your ears. Try tweaking Early and Diffusion vs. the reverb parameters when placement is in the front, to get the right perspective vs. being in the rear (full verb). It’s pretty much an ear process rather than theory books matter.
You should try feeding a full mix to Eclipse and play with these parameters to understand how it moves your listening position in the room.
Can’t get more practical than that!