Frame pacing is a technique employed on the driver level (and possibly hardware too) to make the frames be rendered at an even pace, every so many milliseconds. For example if your GPU is running a game and renders 60 frames in 0.2 seconds and freezes for the other 0.8, technically it is still running your game at 60 frames per second, although this scenario is an exaggeration and the game would be unplayable.

Ideally to run perfectly at 60fps your GPU should render each and every frame at exactly 16 miliseconds, no more and no less, in order to obtain a perfect 60 frames over 1 second. Many hardcore gamers turned over to NVidia because NVidia's drivers and GPUs produced very smooth and close to perfect frame pace during gameplay, unlike AMD's Radeon's that jitter anywhere from 8 to 80 milliseconds while rendering frames, even though they produced the same 60 frames in a second as NVidia did.

I agree that 99% of the people will not even notice this effect even on a blind test, but hardcore gamers frowned upon AMD's GPUs because those milliseconds lost meant ever-so-slight visual stutters that can get them killed when in professional tournaments. So AMD tweaked (or is tweaking) their drivers in order to make them more evenly paced when rendering frames.

Now... wasn't that a nice story telling moment?