A pawn island is a group of pawns of the same color that is completely separated from the player’s other pawns by at least one empty file on each side, with no friendly pawn on an adjacent file.
The more pawn islands a player has, the more fragmented their pawn structure becomes. For instance, if White’s pawns sit on the a-, b-, d-, and g-files, they form three separate islands: {a, b}, {d}, and {g}. Each isolated group is a potential target, since the pawns within one island cannot assist those in another.
In practice, counting pawn islands gives you a quick read on structural health. Fewer islands generally means a more compact, easier-to-defend structure. Aim to avoid exchanges that split your pawns into multiple disconnected groups, and try to keep your pawns connected — meaning placed on adjacent files — whenever possible.