Plain English Meaning
An arm measurement that comes out negative because the item being weighed sits in front of the airplane's chosen zero point. The minus sign just shows direction relative to that zero point, not that something is wrong.
Full Definition
In weight and balance computations, a value expressed in inches that is negative because the reference station (arm) lies forward of the datum. Since arm is measured in inches from the datum, any station ahead of the datum carries a negative sign, and the resulting moment (weight × arm) is also negative.
Context Anchor
Seen in aircraft weight and balance problems when listing an item’s arm, which is its distance from the reference datum.