Insufficient material is a situation in which a player no longer has the pieces needed to deliver checkmate, regardless of how the position plays out. The game is immediately declared a draw under the official rules.
Under FIDE rules, insufficient material is automatically recognised in these specific configurations: king versus king alone, king and bishop versus king alone, king and knight versus king alone, and king and bishop versus king and bishop of the same square color. In all these cases, it is mathematically impossible to force checkmate, even if the opponent makes the worst possible moves.
In practice, it is important to distinguish between automatic insufficient material — immediately recognised and enforced by chess software and arbiters — and certain theoretically drawn positions not covered by this rule (for example, two knights versus a lone king, where checkmate is theoretically possible but cannot be forced against perfect defense). Knowing these distinctions will prevent you from playing on needlessly or, conversely, claiming a draw incorrectly.