Plain English Meaning
The little ball in the cockpit instrument is sitting right in the middle, which means the airplane is flying straight through the air without sliding sideways.
Full Definition
A condition in which the ball of the inclinometer (the slip-skid indicator) rests in the middle of its glass tube, between the two reference lines, indicating that the airplane is in coordinated flight with no sideways slipping or skidding.
Context Anchor
Seen on the instrument panel during stall practice, slow flight, and turns when checks that use is keeping the airplane .