D+/D- is used in conjunction with the Directional Movement Index. The DMI indicator is made up of three plots; the difference between today's high and yesterday's high is the up movement or D+, the difference between today's low and yesterday's low is the down directional movement or D-. The D+/D- is the difference between the moving averages of the two. You determine the period over which the moving averages are calculated.

A positive D+/D- (D+ above the D-) indicates an upward trending market while a negative D+/D- (D- below the D+) indicates a downward trending market. In a trend-less market, D+/D- hovers around zero.

References

Wilder, J. W. New Concepts in Technical Trading Systems.