Ok,
if you watch a tom and jerry cartoon, it plays 25 frames of animations every second? So 25 images are drawn to the screen every second?
When you make a game DBP is just doing that. Imagine that your code is generating another world, and that's a world that will exist EVEN IF YOU DON'T SYNC. In that other world you will place cameras, and every time you SYNC that camera will take a snapshot of that other world and render it to your screen.
get it?
Most people will SYNC just before the loop command. FOr example.
your whole code... in pretend... might look like this:
load world
load characters
load weapons
DO
Control characters
control animation
AI
physics
Score
Player Control
SYNC
LOOP
If you think back to Tom and Jerry, the code is like the animator, drawing the individual frames onto the page, and deciding what is in each frame. the sync command just plays that image to the screen. If you are rendering 60 frames in a second, you will obviously obtain smoother animation than if sync the camera only 5 frames per second.
I hope this is makine sense. If you tell DBP to set the sync rate at 100FPS, it will update the screen every o.o1 seconds, and not faster. You are effectively telling it, not to update the screen any more that 100 times in a second. This is why, if you do not limit the Sync rate, DBP will update the screen as many times as it can, meaning you could have an FPS up to anything.
