@DamalaniSingh: I wonder why you see the need for a bounty when web pages such as this give a good explanation already. In order to avoid a link-only answer, I will summarize the LEX-related gist here (LEX = leading edge extension).
You may know that a delta wing forms a powerful vortex on its suction side at high angle of attack which allows it to work well past angles of attack where conventional wings with less leading edge sweep have stalled. Adding what is in essence a small delta ahead of the main wing will give you the benefit of both: Better L/D and overall more lift at low angle of attack, plus much improved high-alfa capabilities.
The mechanism is the same as for regular delta wings, but the vortex will do its magic not only on the LEX, but also on most of the wing behind it. First a picture of CFD simulations, taken from the Aerospaceweb page mentioned above:
Now proof that the same happens in the real world, again copied from the Aerospaceweb page:
Note the wool tufts on the LEX and wing: The flow over the wing is really separated, because the tufts point in all directions except backwards. What is also visible is the disadvantage at high angle of attack: The vortex bursts behind the wing, which causes extreme vibrational loads on the tail fins. They needed to be reinforced at their root to extend the structural life of the airframe. Again, also the last picture of this answer is copied from the Aerospaceweb article. You should read it yourself!
@VictorJuliet: And lastly, please give the bounty to Pawel! His answer was first, and it is correct. I have enough reputation already.