What Is Meant By What Direction ?

Imagine a snapshot of a machine operating. In the snapshot, imagine being able to see arrows drawn at each bearing indicating what direction that bearing was moving in at that moment in time. From the snapshot, you can thereby determine if the bearings are moving in the same direction at the same time (in unison or 'in phase') or not.

Phase helps determine how different locations on the machine (different bearings, usually) are moving relative to one another