As happens too often in the API docs, they give a technically correct but superficial definition and expect you to figure it out on your own. If you care, when a plane is represented as
[Point3d, Vector3d]
The point is a location on the plane, and the vector is the normal to the plane.
When a plane is represented as a four-element array, the first three values are the x,y,z components of the normal vector to the plane, and the fourth value is the distance from the origin to the plane along the normal vector. Face#plane returns this version.