Frame pacing describes the time it takes between displaying frames. You want the frame rate to be stable (always 30, 60 and120) but you also want the frames to be happening at the same interval.
For example, 60FPS is ideal for most people. What happens when all 60 frames are displayed in the first half of a second? Then something happens and nothing is displayed at all for the second half of the second? You get stutters!
Vertical Sync allows the game to synchronize the frame rate of the game with the monitor refresh rate for better stability.
Please keep in mind that enabling both options within the Moonlight client will increase latency by a small amount.