I've been searching the net for a technical explanation of what ISO settings mean
I'm not certain what you mean by "mean" - if you are interested in the "inner workings" rather than the definition of ISO there are some very good sources of information right down to the level of the semiconductor physics.
Just at the moment I forget the very best resource (a paper analysing the differences between CMOS and CCD sensors, IIRC), but I think I found it by following links from
which of itself is a bit closer to the "effect on the picture" than you may be looking for, but includes some hard facts on the low level practicalities.
Astrophotography resources tend to be good on this stuff, as the image creation and physics knowledge are often reasonably well balanced.