Phase is the relationship of the voltage levels on the rails.
In Analog, two conditions determine the direction of the locomotive: Amplitude and Polarity. The amplitude of the voltage determines speed, and the polarity or direction of current flow controls the direction of movement.
In Digital Command Control there is no concept of polarity. The decoder controls speed and direction. The PHASE of the signal on the rails is important, as incorrect wiring will cause a short circuit. No negative voltages are present on the rails, they are either logically high or low. DCC does not use a negative voltage on the rails.
Phase Sequence is important. In a reverse loop Rail A eventually connects to Rail B. Both rails must maintain the correct phase relationships, in this situation a short circuit results, as one rail is energized and the other is at "0" potential. To prevent this, the loop must be isolated via gaps in the rails and an autoreverse device used to maintain the phase relationship as the train returns to the mainline.
This is very important with booster districts: Both boosters must be in phase to ensure trouble free operation as trains move across the gap from one district to the next.