Normally, a coins design is upright on both sides when the coin is flipped over vertically.

A ROTATED DIES error is when the reverse die is not aligned properly with the obverse die.  This type of error can be easily identified by holding a coin at the 3-o-clock position between your thumb and forefinger, looking at the obverse design upright, and flipping in over vertically.  If the design on the reverse isn't upright (looks rotated either clock-wise or counter clock-wise), you've found a rotated-dies error.

Some rotated dies can be quite dramatic, some even a perfect 180 degrees!  With some searching, they can be found in pocket change.