Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4. e3/4...b5/5. Nxb5/5...cxb5/6. Qf3/6...Qc7

6...Qc7
This move tries to cage White's Queen on a8. White's queen will soon get trapped. Black is guarding b8 and b7 with his queen so that Bb7 is possible after Qxa8. Of course, the only move is Qxa8, as else white will lose a piece for nothing.