a group of connected friendly pawns. In the diagram Black has three pawn islands to "White's" two. It is usually considered to be advantageous to have fewer pawn islands than the opponent. Two pawn islands to Black's three

A group of pawns separated from other pawns of the same color. It is generally hard to defend a lot of pawn islands. ( Diagram)

A group of pawns of the same colour separated from the next pawn by at least one open file. More islands indicates a weaker pawn structure.