It is considered as another way to produce lift, and if this is attached to the wing then it is called a leading-edge extension. But, why put an extension on the wing? What kind of benefits does it bring to the plane?
The leading edge extension creates vortices in a controlled way. It improves the quality of flight characteristics by changing the airflow behavior over the wing, reducing the pressure on the upper surface of the wing and this change increases the lift.
A wing strake has a sharp leading edge, and when the plane pitches up, reaching a different pitch angle, the airflow separates from the leading edge, however, due to the swept wing, the separated flow forms a circular pattern on the upper wing.
The leading-edge extension (LEX) delays the stall of the plane increasing the angle of attack of the wing in which the plane stalls, and this is great for maneuvering at low speeds. The first airplane to use this device was the Northrop F-5.
Some aircraft that have these leading edge extensions are:
F-16, F-18, Sukhoi Su-27, the concorde, Tu-144, among others.
In some pictures or videos, it is possible to notice the vortex condensation when maneuvering at high angles of attack, an example common in an aerial battle, take-off and landing.