Basically it has to do with the conception that everything in the universe is in motion, ergo there is no ‘absolute reference frame’. That is to say, anytime we make a measurement of something the value we get is with respect to the contextual frame of reference we’re measuring from—i.e., we’re also moving at some given rate of speed ourselves.
A straightforward example of this would be to think about what the speed of a car driving 55mph looks like from different reference frames. If you’re traveling alongside them at 60mph, then from your perspective they’re traveling -5mph relative to you. But from the perspective of a stationary (~relatively speaking!) person at the side of the road, that car is driving 55mph.
There are some caveats for the speed of light, however—see here for more: http://en.wikipedia.org/wiki/Special_relativity#Lack_of_an_absolute_reference_frame
And the more spectacular, and useful parts of chemistry, thankyouverymuch.
Does the math on this hold up with the M-M experiments? I guess that's the question I've been trying to ask.
/johnny