In all modes you can set the number of beats per bar and the number of bars. The two are multiplied together to give a quantum. So 4 beats per bar times 1 bar = a quantum of 4. All Ableton Link-enabled apps use this quantum to ensure that any apps with the same quantum are in phase with each other.
You could think of it like a phrase length. If you had an eight bar phrase with four beats in a bar, you'd probably want all Ableton Link-connected apps to be at the same point in the eight bar phrase. So you'd set bars to 8 and beats in a bar to 4.