Reference frames in relativity?
It depends on your definition of "layman" but Patrick Wallace has a slide presentation on what changed and why with the new reference systems. There is also the USNO Circular 179 which covers all of this and is pretty readable. 

In a lot of ways it boils down to the realization that the wobbly (in space and rotation) Earth was a lousy thing to base co-ordinate systems on. The new systems are what's known as "kinematically non-rotating" which mean they are fixed in space and don't move. This makes it much easier to make more precise theories (which were needed) to describe how things like the orientation of Earth's pole (the precession and nutation you may have heard about) move in the Solar System without having to reference it to something like the equator and ecliptic which are always moving about.