Gyro at position A has its spin axis aligned with true north. The gyro will maintain its alignment in space, the direction of true north relative to the gyro will change as the earth rotates and the observer's position in space moves through B, C to D. This is apparent drift. It is zero at equator and maximum at the poles.
The undesired movement of a spin axis from its original position. Gyro wander can be apparent or real. Apparent gyro wander is the result of the movement of the earth and is as shown in the illustration. Real wander can result from mechanical and/or electrical reasons, usually caused by friction in the gimbal bearings.