The Directional Momentum Indicator is an attempt to quantify the trending or directional behavior of a market. It helps identify trends and whether or not price is moving quickly enough to be worth a long or short play.

+DI and -DI are components in the calculation of the ADX (Average Directional Movement) and ADXR (Directional Movement Rating) indicators. Perhaps the best way to think of the +DI is as a measure of the percentage of upwards movement. When the +DI value is greater than the -DI, a long position is indicated.

Information provided by Charles LeBeau's Technical Traders Guide to Computer Analysis of the Futures Market.