In the diagrammed position, the Black pawn on the c6 square is backward. Backward pawns are usually a positional disadvantage, since they are hard to defend. Also, the opponent can place a piece, usually a knight, on the hole in front of the pawn without any risk of a pawn driving it away. The backward pawn also prevents the black rooks and queen on the eighth rank to attack the piece placed on the hole.
If the backward pawn is on a half-open file, as in this case, the disadvantage is much larger, as it can be more easily attacked by an opponent's rook or queen on the c-file. Pieces can become weak when they are devoted to protecting a backward pawn because they are obligated to defend the pawn and cannot be developed for other uses.
Modern opening theory features several openings in which one of the players deliberately takes on a backward pawn in exchange for some other advantage such as the initiative or better development. An excellent example is the Sveshnikov variation of the Sicilian Defence. After the moves 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 (or 4...e5!? 5.Nb5 d6 - the Kalashnikov Variation) 5.Nc3 e5!? 6.Ndb5 d6 (Diagram above), Black has a backward pawn on d6, but White now has to endure a displacement of his knights and an undermining of his center after 7.Bg5 a6 8.Na3 b5 9.Bxf6 gxf6! 10.Nd5 (dodging the threatened pawn-fork of the knights) 10...f5! [or 10...Bg7 11.c3 (facilitating the a3N's return to the center by the route Na3-c2-e3) 11...f5!) 11.c3 Bg7 etc.