Dynamic Time Warp through SPTK tool
The documentation clearly says (emphasis mine):


  If –s option is specified, the score calculated by
  dynamic time warping, that is, the distance between the test data and the reference data is output and sent to Scorefile.


That means that your decimal number, $0.678382$, is the actual DTW distance between your 2 speech samples.

Now DTW is a distance measurement, and as any distance it only gives you relative information. For example, you could set a treshold and says that under this treshold, the 2 signals correspond to the same word.  If you apply DTW on a pitch contour, you could determine whether the 2 melodies are similar (that's used in query by humming).

Of course, the physical meaning of the distance depends completely on the one of the time series compared : if you compare voltage values, or frequency values, etc.
In your case, you compare normalized volumes mapped to unsigned 8-bit data.

I advise you to learn more about DTW and to try to analyse what the distance actually tells you, and how that relates to your application