Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4. e3/4...b5/5. a4

5.a4
This move tries to attack Black's queenside and lure it to it's doom. b4 is almost universally played after this, as if Qa5, the queen is exposed. b4 tries to boot the knight into oblivion and disrupt White's queenside.