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Hi Robert,
Put simply, stacking images reduces noise by averaging it out. The more images in the stack the greater the level of noise reduction. I'm sure someone who knows a lot more than I do about it will expand on this.
A good way I was helped to understand it is. Imagine you draw a 5 x 5 grid (representing pixels) on some clear plastic and colour in the squares but randomly leave say 8 clear (this represents noise). Now do it again but leave a different lot of 8 squares clear. Do this say 5 times and then stack these grids on top of each other. Due to the random spacing of the clear squares the stacked image would be almost completely (if not completely) coloured with almost no noise (clear squares). The noise was averaged out due to its random spacing.
I hope this helps
Cheers
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