12CPR encoder

Excuse me all senior,

I am working with 12VDC Precision Faulhaber Model 2342L012CR. I try to count RPM of my motor, but I am new with arduino.

motor specification:
Working voltage : 12V
No load speed : 8100RPM
RPM : 120RPM
Nominal output power : 18W
No load current : 75mA
Load current : 1400mA
Gearbox ratio = 1 : 66
Encoders : Optical
Encoder Phase : AB
Encoder Resolution : 12CPR

Could you please explain me briefly about the calculation, and I also need you sketch.

thank in advance