If the Earth were flat, we could describe the longterm motion of a plate as some straight-line direction (e.g., NE or 53 degrees from due north). But because of the curvature of the Earth it is not that simple. Think, for example, how a plate may drift over the north or south pole. How, then would you describe its direction?
One can describe the motion of a plate (or of a point) on a spherical Earth by way of a pole of rotation or euler* pole (commonly pronounced "oiler"). Imagine an euler pole as a pivot point. A plate moves about that pivot point with a set angular speed. It's like a dog chained to a peg anchored into the ground if it just runs around in a circle at the end of its chain.
The euler pole that defines the motion of a plate can be defined from the orientations of fracture zones. The fracture zones themselves are flow lines. They show the direction(s) that a plate moved through time. Arcs drawn perpendicular to the fracture zones on a given plate will intersect at the plate's euler pole. (see Fig. 5.3 in Kearey and Vine)
* Leonhard Euler was an 18th century, Swiss-born mathemetician