Plain English Meaning
When the datum (the zero point an aircraft measures everything from) is set somewhere on or behind part of the airplane, anything in front of it has a negative distance. Those negative distances are called minus inches, and they pull the balance calculation in the opposite direction from items behind the datum.
Full Definition
An informal weight-and-balance term used when an arm or a station is located forward of the reference datum, giving it a negative value expressed in inches. In moment calculations, weights at these stations produce negative moments, which are subtracted from the total moment when finding the center of gravity.
Context Anchor
Seen in weight and balance tables and calculations when a station is located forward of the aircraft’s reference datum.