I was just guessing the best move in the given position(black to move) . I thought it would be 'Be6' , But the engine gave 'c5' , which weakens the 'd5' square . I want to know the plan behind the move 'c5' if it is the best move.
When I play a move and the engine says some other move is better (which is to say, all the time), I play my move to see what the punishment is going to be. Once I understand why my move isn't good, I back the game up and play the engine's suggestion. Usually it's pretty clear.
In this case it isn't. This is one of those cases where the engine is selection a wholly inhuman move in order to shave a few fractions of a pawn from the position's evaluation.
This variation, Black holds on to his material and thus his chances. One Bishop is superb, the other is better than White's bad white-squared bishop. His Rooks are on the open files. His Knight is prepared to leap onto
d4, a move made possible by the anti-positional
[White "Stockfish 5"] [Black "R K"] [Title "1... c5 variant"] [FEN "1rb2rk1/5pb1/p1pp1np1/q6p/2PBP2P/2N2P2/PP1Q2P1/3RKB1R b - - 0 1"] 1... c5 2.Be3 Nd7 3.Be2 Re8 4.Kf2 Nf8 5.Rc1 Ne6 6.b3 Qd8 7.Nd5 a5 8.Nc3 Bb7 9.Nb5 Be5
Stockfish rates White's advantage at .7; that is, about 70% of a pawn.
1... Be6, the engine falls into a more human-looking mode of play. Be6 does little and gives Black's pieces no synergy. He presents White with few challenges. In many lines, Black exchanges Queens on d2 because he's got nothing else. When Black is down a pawn and White's King is exposed, to exchange Queens is to beg for a draw. In this line, Black's constricted position leaves him no dynamic moves and he settles for trading off most of his pieces.
[White "Stockfish 5"] [Black "R K"] [Title "1... Be6"] [FEN "1rb2rk1/5pb1/p1pp1np1/q6p/2PBP2P/2N2P2/PP1Q2P1/3RKB1R b - - 0 1"] 1... Be6 2.b3 Rfd8 3.Na4 Qc7 4.Qf4 Qe7 5.Kf2 c5 6.Ba1 Bd7 7.Nc3 Bc6 8.Be2 Ne8 9.Nd5 Bxd5 10.exd5 Bxa1 11.Rxa1 Nf6 12.Rae1 Nd7 13.Qg5 Re8 14.Qxe7 Rxe7 15.f4 Rbe8 16.Bf3 Kg7 17.Rxe7
...with a score of +1, if you believe such things.
All that being said, Black is going to have to work awfully hard to win back the fraction of a pawn. I am sure the way is fraught with peril. Perhaps this is the price of dropping a pawn.
The move c5 is obviously anti-positional. So the purpose must be tactical.
[Title "Anti-positional c5"] [FEN "1rb2rk1/5pb1/p1pp1np1/q6p/2PBP2P/2N2P2/PP1Q2P1/3RKB1R b - - 0 1"] 1... c5 2. Be3 Rxb2 3. Qxb2 Nxe4 4. Bd2 Nxd2 (4... Bxc3 5. fxe4) (4... Nxc3) 5. Kxd2
1…c5 2.Be3 Rxb2 3.Qxb2 Nxe4 is the obvious try. But after 4.Bd2 the position is pretty tricky because 4…Nd2 fails to 5.Kxd2, Bxc3 fails to fxe4 and Nxc3 steps into a selfpin. But the selfpin cannot be exploited by white because black has Re8+/Qe5+ coming up. This looks like a pretty crushing attack.
If white prefers to exchange the Bd4 for the Nf6, then the weak d5 square is more than balanced by the weak black squares in whites position. Bd4 is a much bigger threat than Nd5.