A pawn island is a group of one side's pawns connected along adjacent files but separated from that side's other pawns by one or more empty files.
Count the clusters: pawns on a2-b2-c2 and f2-g2-h2 form two islands, split by the empty d- and e-files. As a rule, the fewer pawn islands you have, the healthier your structure, because pawns within an island can defend one another while isolated groups must be guarded by pieces.
More islands usually means more weaknesses — the pawns at the edges of each island are harder to defend, and the open files between islands give enemy rooks routes in. A single connected mass of pawns is the ideal; three or four scattered islands often spell long-term trouble.
Pawn-structure decisions, including which captures to make, often come down to island count. When recapturing, choosing the option that leaves you with fewer islands is a sound default.
Pawns in the same island can defend each other, while separate islands must be guarded by pieces and the open files between them help enemy rooks. Fewer islands means fewer weaknesses to babysit.
Count each group of your pawns that sit on adjacent files with no empty file inside the group. A gap of one or more empty files starts a new island.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.