Plain English Meaning
The pushing and pulling forces the air creates on an aircraft as it moves. They are what make it climb, descend, speed up, slow down, or stay level.
Full Definition
The forces produced on an aircraft by its motion through the air. The four primary aerodynamic forces are lift (acting upward, perpendicular to the relative wind), drag (acting rearward, opposing motion), thrust (acting forward, produced by the powerplant), and weight (acting downward due to gravity). In steady flight these forces are in balance; any change in airspeed, attitude, or configuration alters them and changes the aircraft's .