Agility in tactical aircraft maneuverability became a key design search during the 1970's, as it became clear that at high speeds in air-to-air combat, the aircraft that could turn inside the other would have the combat advantage. Of course this was always the case even during WW I and WW II (viz. The Japanese Zero).

Northrop F-5A without LEX
( John Shupek collection )

One of the ways to get agility is through lightweightness. At high speeds though, designers realized something more was needed; this lead to the Leading Edge Extension or "LEX". You can see the evolution of the LEX by comparing the F-5A with the YF-17. It is that part of the wing extending from the inboard wing forward along the fuselage nose section.

Northrop YF-17 with LEX
( John Shupek collection )

The LEX enables higher sustained turn rates and in general improves handling and lift as the aircraft reaches angles of attack at which it would otherwise stall. The LEX can be pictured as effectively grabbing air and controlling the flow to generate lift while, outboard, the wing may be stalled.