Basically when two pickups are wound in the same direction, both have the same magnetic polarity towards the strings, the beginning of each coil is grounded and the finish of each coil are positive. The two pickups are considered "in" phase with each other. You need at least two pickups in an instrument to determine if the pickups are in or out of phase with each other.
A pickup can also be out of phase with it self because of incorrect wiring or connections within the pickup. In question 450 the example shows the normal and correct way of wiring a standard humbucking pickup. If the beginning and finish wire from one bobbin was reversed, the pickup would be electrically out of phase.
WRITTEN ON JUNE 12, 2015, BY