Chess Opening Theory/1. e4/1...c5/2. Nf3/2...d6/3. d4/3...cxd4/4. Nxd4

White recaptures the pawn, and centralises their knight. Their knight is active and centralised, and they have the space advantage and open lines to develop both of their bishops. Black meanwhile has the advantage of having both d- and e-pawns.

If White were to move again, they would love to play the move 5. c4. The structure with pawns on c4 and e4 is called te "Maróczy bind" and makes it very hard for Black to achieve ...d5. Then White could play Nc3 without blocking the progress of their c-pawn.

As a result, 4...Nf6 is almost always played. This gains time on the e4-pawn, asking White how they wish to defend it. 5. Nc3 is the most common way to defend it: developing a piece, but preventing White playing c4 for now. If White insists on getting the Maróczy bind, they may instead try 5. f3, the Prins variation.

1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4

Wikipedia has related information at Sicilian Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.