Official Aviation Definition
Slats are high-lift aerodynamic devices mounted on the leading edge of fixed-wing aircraft. When deployed forward and downward, they open a slot allowing high-pressure air from beneath the wing to flow over the upper surface, delaying boundary layer separation and allowing flight at higher angles of attack.
In Simple Words
Panels on the front edge of the wings that slide forward to help the plane fly safely at slow takeoff and landing speeds.
Real-World Flight Example
When the pilot selected Flaps 1, the leading edge slats deployed automatically to expand the low-speed flight envelope.