Plain English Meaning
It is the actual line-of-sight distance from the aircraft to the station on the ground. Because the aircraft is up in the air, that line is always a little longer than the distance you would measure on a map.
Full Definition
The straight-line distance between an aircraft and a ground station, measured directly through the air rather than along the ground. Because the aircraft is above the station, this line forms the hypotenuse of a right triangle whose other sides are the aircraft's altitude and its horizontal distance from the station. DME (Distance Measuring Equipment) displays slant-range distance, not the horizontal distance over the ground.
Context Anchor
Seen when using DME, especially when comparing the DME readout with position on a chart.