what?
You can have 1000 fps on a 60hz monitor.
You can't actually see more frames than 60 (ON A 60HZ MONITOR), but the frames will be more up to date if you have more fps.
Meaning more fps = more better
no matter what hz your monitor is.
Freesync will sync your monitors hz dynamically to your fps, and will only work below 144hz.
Meaning, if you get 40fps on a 144hz monitor, you will feel more lag and spikyness than 40 fps in a 40hz monitor
So what freesync does is say that same scenario, it will make your monitor 40hz, just pair with fps of the monitor, but it won't make it go above 144 (cause that doesn't make sense.)