Iím sure someone will chime in with a more detailed answer, but it means it doesnít add up. Some ions are positive and some are negative and they have to balance. If they donít it tells you your water report is off somewhere.
I donít think itís a big deal. The big thing is the alkalinity and what you might have to do get your mash and sparge pH into the ideal range. Your water isnít terrible, but it needs a little help, especially for pales.