Simply, it means not reading a reading on the same plane as the
indicator. In the case of a speedometer, you read PERPENDICULAR to
the measuring apparatus. In other words, immediately above the
reading. (Not vertically above, speedometer is slanted). To correct
parallax error, what some instruments have is a mirror. If the
needle is seen in the mirror, it shows that there is a parallax
error because of the laws of reflection, blah, that I won't
explain. I would need a diagram. If only I can upload a simple
diagram, 
.