White's move 2. Nf3 increased her control of the d4 square, enabling her to play d4 next move. Black cannot hope to resist White's plan any further, and should go about his own development.
2...d6 is the main line move, controlling e5 for a potential advance of the e-pawn and letting the queen's bishop out, while also holding down the c-pawn in case White should go down a different path.
2...e6 is an older move freeing the king's bishop although d6 will often follow soon.
2...Nc6 reserves options in the centre and keeps an eye on d4, as does 2...g6 planning Bg7, and 2...a6 can restrain some of White's longer-term plans and provide the springboard for a b5 advance, though it neglects the centre.
There are many slightly different Sicilians with similar plans for Black, in which various combinations of these moves may turn up in almost any order.
The one significant departure from the main lines is 2...Nf6, evoking Alekhine's Defence by attacking the e-pawn to tempt it forward.