Sicilian Najdorf

I don't get 3. d4 nor 5. a6, could someone please explain them to me?

5...a6 prevents white from getting either knight or his white bishop out to b5; it also sets up Pb5 later so that black can get a good queenside attack going.

3. d4 keep black from controlling the center (the d4 square) with that c5 pawn; after the exchange, white gets to occupy that square with his knight, and sets him up for the yugoslav attack, which can be quite strong against the sicilian

3.d4 willingly gives up the central pawn in order to gain a lead in development and somewhat weaken black's central stake.

5...a6 is played to keep bothersome white pieces out of b5 where they could cause trouble in black's position.

Ok, thanks a lot