Chess Opening Theory/1. e4/1...c5/2. Nf3/2...Nc6/3. d4/3...cxd4/4. Nxd4

= Open Sicilian with 2...Nc6 = Here White is moving a piece that is already developed, so they are playing an opening move that does not help their development. They are capturing a pawn, however it is anticipated that Black can capture White's pawn when White played 3.d4. In return for neglecting development White is getting a centrally located knight, and Black neglected development when they made two moves with the same pawn.

A good point here is it is bad for Black to trade knights on d4, and it also bad for White to trade knights on c6. If Black plays 4. ... Nxd4 followed by 5. Qxd4 White will have better control of the center and Black cannot easily chase away the White queen because the Knight that was on b8 has been traded. Also if White plays 5.Nxc6 then black can play 5. ...bxc6 which will strengthen Black's center and prepares for the d7-d5 push.

Theory table
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4