x6gas

I've also done this sort of thing to an audience of non-experts.  I tend to cover the broad principle of correcting for the Earth's rotation which leads nicely on to guiding (nothing too technical, just an explanation that you point at a star you're not imaging and movements of that star are used to correct for the movement of your target).

Then I've done a bit on the principle of digital stacking - again in simple terms pretty much as Carole suggests.

I have done a bit on narrowband imaging and I think have just about got away with explaining the physical principles behind emission nebulae with the help of a diagram or two and how that translates.

I agree that people always seem interested in how far away things are, and of course that gives the chance to say that when the light hitting my camera left this object then dinosaurs were roaming the Earth (or whatever)...

Have fun with it!