In chess, an isolated pawn is a pawn for which there is no friendly pawn on an adjacent file. Isolated pawns are weak for two reasons:
1. Pieces attacking them usually have more elasticity than those defending them. In other words, the attacking pieces enjoy greater freedom to do other things (threaten to win pieces, checkmate, etc.), while the defending pieces are restricted to the defense of the pawn. This is because a piece that is attacking a pawn can give up the attack to do something else whereas the defending piece must stay rooted to the spot until the attacking piece has moved. The defending piece is thus said to be "tied down" to the pawn.
2. The square immediately in front of the isolated pawn is weak because it is immune to attack by a pawn (often providing an excellent outpost for a Knight). Thus an isolated pawn provides a typical example of what Wilhelm Steinitz called weak squares.
An isolated Queen pawn (IQP), called isolani is often a special case. An IQP is one in the Queen's file (d file). Assuming that it is a White pawn on d4, the weakness of such a pawn consists in its having to be defended and the weak square in front of it (i.e. d5 for White) being of particular importance. However the presence of open files in the important King and Queen Bishop (e and c) files as well as the outposts at e5 and c5 enable the player with the IQP very favourable attacking chances in the middle game. Once the game reaches the endgame, the isolated nature of the pawn becomes a greater weakness than these strengths. Therefore the player with the IQP must take advantage of the temporary strength before an endgame is reached. Sacrifice of the pawn by white and blockade of the pawn by black are common themes.
Source: "Wikipedia, The free Encyclopedia"