Plain English Meaning

Movable slats are small panels along the front edge of the wing that slide forward when needed. The gap they open lets air flow over the top of the wing more smoothly, so the wing keeps lifting the airplane at slow speeds and steep climb angles instead of stalling.

Full Definition

Movable slats are leading edge high-lift devices consisting of small aerodynamic surfaces mounted on the front of the wing that can extend forward and away from the leading edge to form a controlled gap. At high angles of attack, this gap allows higher-pressure air from below the wing to flow through to the upper surface, re-energizing the boundary layer, delaying airflow separation, and allowing the wing to keep producing lift at slower and steeper than it otherwise could.