A steady series of 12 taps means turn right at the intersection youre approaching; three pairs of two taps means turn left. Not sure what your destination looks like? Youll feel a vibration when youre on the last leg, and again when you arrive.
Source: http://help.apple.com/watch/?lang=en#/apdea7480950
Source: http://help.apple.com/watch/?lang=en#/apdea7480950