Pawn Islands
A pawn island is a group of pawns that stand on consecutive files, with no friendly pawn on either side of the group. Any open file -- a file with no pawn of that color on it -- splits the pawn mass into a new island. A side with all eight pawns still on their starting files has exactly one island; trade off a few pawns in the wrong pattern and that one island can fracture into two, three, or more.
Islands matter because every island needs its own defenders. A single pawn stranded on its own island has no neighbor to trade off weaknesses with and no neighbor to eventually support it, so it tends to become a long-term target that a rook or king can attack for the rest of the game. Two or three compact islands are usually fine; four or five small islands is often a structural disaster, even if no single pawn is lost yet.
This is one of the first things strong players glance at when sizing up an endgame: count the islands on each side. All else being equal, fewer islands means fewer weaknesses to defend and fewer targets for the opponent's king and rooks to chase.
Worked examples
Two pawn islands
The lone a5 pawn is stranded on its own island; the c3/c4/d2 cluster forms a second, larger island. Two islands total.
Four pawn islands
Pawns on a2, c2, e2, and g2 -- every file between them is completely open, so each pawn is stranded as its own island. Four islands from four pawns is about as fragmented as a structure gets.
One healthy island
b2, c2, and d2 sit on consecutive files with no gap between them -- a single compact island, the healthiest shape a pawn group can have.
Test yourself
This structure has split into two islands. Click every pawn (all four).
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