FPS (Frames Per Second) is the number of screen updates your computer can handle in one second and the value returned depends on the specification of the computer the program is run on.
The Sync Rate command allows you to 'cap' the refresh rate, so using
Sync Rate 60
will restrict it to approximately 60 frames per second on all machines where it's capable of being much higher. Obviously, on machines not capable of 60 fps then this command is of no use and the actual rate will be lower.
Sync Rate 0
turns off this capping and it will max out at whatever rate the machine is capable of.
I would suggest that 60 is a reasonable value to aim for if you want reasonably smooth action, though where you want a game to run the same speed on all machines, you should use Sync's based on a timer.
TDK_Man