I've always gone with the meditation chamber idea. And, yeah, it is just the helmet, not the mask, if you consider the figure based off of it to be accurate. There's a meditation egg playset that has Vader with removable helmet but not mask (as opposed to the standalone figure that removes the helmet and mask in one piece. So, logically, that's what it is. In terms of the movie, there are actually two opinions on the commentary about why this happens. According to Lucas, it's so we can see that Darth Vader is indeed human and not a robot, so his revelation at the end of the movie makes sense. According to Kershner, though, immediately after that clip, he said that it was to add mystery and ambiguity to Vader. "Is he human? Is he an android? We don't know!"