By playing 4.c4, White gains some space at the expense of not being able to defend the d-pawn. This move also tries to destabilize the d5 pawn, and open up a line for the bishop.

Common responses by Black include Nf6 which guards the pawn and develops a knight, c6 also guarding the pawn, just taking the c4 pawn with dxc4, or Bb4+ to gain a tempo on the white king and develop a bishop.

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Wikipedia has related information at French Defence.

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