what exactly is a polar shift?

The 'polar shift theory' is the hypothesis that the axis of rotation of a planet has not always been at its present-day locations or that the axis will not persist there; in other words, that its physical poles had been or will be shifted. The Pole shift hypothesis is almost always discussed in the context of Earth, but other solar system bodies may have experienced axial reorientation during their existences.

Pole shift hypotheses are not to be confused with plate tectonics, the well-accepted geological theory that the Earth's surface consists of solid plates which shift over a fluid asthenosphere; nor with continental drift, the corollary to plate tectonics which maintains that locations of the continents have moved slowly over the face of the earth,resulting in the gradual emerging and breakup of continents and oceans over hundreds of millions of years.

Pole shift hypotheses are also not to be confused with Geomagnetic reversal, the periodic reversal of the earth's magnetic field (effectively switching the north and south magnetic poles). Geomagnetic reversal has more acceptance in the scientific community than pole shift hypotheses.

It is now established that true polar wander has occurred at various times in the past, but at rates of 1° per million years or less. However, in popular literature many theories have been suggested involving very rapid polar shift.