You have defined the motors here
// Motor definitions to make life easier:
#define MOTOR_A 0
#define MOTOR_B 1
// Now spin both!
driveArdumoto(MOTOR_A, CW, 255); // Motor A at max speed.
driveArdumoto(MOTOR_B, CW, 255); // Motor B at max speed.
The code 'instructs' motor_A (value 0 as defined above) to spin clockwise at full speed. Next it 'instructs' motor_B (value 1) to do the same.
When using #defines, the pre-compiler replaces all instances of MOTOR_A by the value that you specified. Same for MOTOR_B, CW and CCW.
Is it loading a 0 to the first digit and 1 to the second?
Not quite sure where 'digit' comes from? But after pre-compiling, your code is
// Now spin both!
driveArdumoto(0, 0, 255); // Motor A at max speed.
driveArdumoto(1, 0, 255); // Motor B at max speed.