Plain English Meaning
It's when a pilot tries to turn the airplane by stepping on the rudder without rolling the wings. Instead of leaning into the turn like normal, the airplane stays roughly level and gets shoved sideways through the sky.
Full Definition
A skidding flat turn is an uncoordinated turn in which the airplane is yawed around its vertical axis using rudder alone, with little or no bank applied. Because there is no horizontal component of lift to pull the airplane around the turn, the tail slides outward and the nose is pushed through the turn by rudder pressure, producing a sideways skid through the air rather than a true banked turn.
Context Anchor
Seen when learning how to a normal turn and keep the airplane , especially during slow or - .