Speed: Bullet to Correspondence Rating:
A high-class waiting move. Black wants to play e5, placing a stake on center control, but first prevents Nb5 or Bb5+. Depending on white's setup, perhaps black will play e6 instead of e5, keeping further control over d5. Black can expand on the queenside with b5 (and possibly b4), in order to gain queenside counterplay.
The Najdorf Sicilian is one of the most popular variations in all of chess. It is highly theoretical, and leads to extremely double-edge positions with winning chances for both sides. White's lead in development means they can often build up a huge kingside attack, while black has lots of counter-attacking chances on the queenside due to their solid center and flexible pawn structure.