Capacitor for current transformer measurement circuit
The explanation is confusing, what RA, RB do is divide the 5 V of the Arduino's supply by a factor 2 to 2.5 V. This is used as a reference voltage because the sensor can generate positive and negative voltages.

Suppose the sensor makes -1 V, by itself the Arduino cannot handle this, you have to be between 0 V and 5 V. But if you "lift" that -1 V by 2,5 you get 2.5 V - 1 V = 1.5 V which is perfectly OK.

But a little bit of AC current will flow, disturbing the 2.5 V reference voltage. This is where the capacitor C1 comes in, it decouples that reference voltage. Indeed, the AC component will now travel (have a path through) the capacitor instead of the resistors. That's what they are saying.

But stating: "C1 decouples the 2.5 V reference voltage" would probably be easier to understand.