The Earth has a magnetic field and two magnetic poles. Polarity is used to describe the location of the magnetic north pole and where it is presently located geographically. Normal polarity is when the magnetic north pole is located at the geographic south pole.
The Earth's magnetic poles have reversed multiple times, and predicting a reversal is highly difficult, as the reversals have no specific length or time frame. These reversals can be seen by studying rock layers, particularly those found mid-ocean, which show minerals that display normal and reversed polarity, depending on which side of the ridge the samples are taken from.