No marketspeak here, but if you're not familiar with the technical details you might be a bit lost. First of all, in order to understand the solution, you need to identify the problem.
The problem is that, currently, refresh rates are STATIC. For example, if I set mine to 60Hz, the screen redraws at 60fps. If I keep vsync disabled to allow my gfx card to push out frames as fast as it can, my screen still only draws 60fps, and screen "tearing" can result as the screen redraws in the middle of the gfx card pushing out a new frame (so I see half-and-half of two frames).
As described, let's say my gfx card is pushing out 25fps. Currently the optimal strategy is to keep vsync off, even though this can result in screen "tearing", because with low fps bigger problem emerges even though screen "tearing" is fixed, with vsync on.
Every time my gfx card pushes out a frame, since vsync is on, it waits to ensure it will not be drawing to the screen buffer while the screen is updating. Since it waits, the screen only draws complete frames. So at 60fps the screen updates in 1/60 second intervals, and the gfx card render at 1/25 second intervals. So, at the beginning of a frame render, the gfx card renders... and the screen redraws twice, and then the gfx card has to wait for the third opportunity to draw before syncing up again. Since it is waiting instead of rendering, I am now rendering at 20fps (since I lose 2/3 refresh opportunities) instead of the optimal 25fps. If I disable vsync, I get tearing, but now 25fps.
This "G-sync" claims to solve that issue by making refresh rates DYNAMIC. So if my gfx card renderas at 25fps, the screen will refresh at that rate. It will be synchronized. No tearing or gfx card waiting to draw.