Black noticed that the move 1…Nd8 right away, fails. White can play 2.Ba4! The point is that this move will defend the white rook at the same time. That is why black first plays 1… b3!
The move b3! threatens bxc2 and at the same time ensures that the move Ba4 won’t defend the rook on c2.
1… b3 2. axb3 Nd8, followed by Rxc6