The Pivot Point is a key item in Array Mesh creation because it sets the center of the operation, thus impacting how the rotation and offset values work to place the instances.
You can define the position of the pivot point by using the X, Y and Z Amount sliders while in Pivot mode. For more interactive manipulation, use TransPose. In this latter case, a little yellow circle will appear at the TransPose line’s starting point. This is the pivot point.
The pivot point is manipulated by clicking and dragging the circle. Because its position is set in the camera working plane, it is advised to switch to an orthographic view and carefully select your point of view before manipulating.
The pivot point is only visible when TransPose mode is enabled. When disabled, the yellow circle will not be seen.