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

5...cxb5
This move bravely accepts White's bold sacrifice. Conventional chess wisdom says "The best way to refute a gambit is to accept it". Therefore, if black would have declined the gambit, white would successfully rip black's queenside apart and Black would just be worse. Now, white's queen will get trapped on a8.

Of course, the only way to try to regain the material is with 6.Qf3.