Exactly-once is the delivery guarantee that each message affects the system once — never lost, never duplicated. It is the hardest guarantee in distributed systems; most infrastructure offers at-least-once (may duplicate) or at-most-once (may drop) instead.
Worked example: true exactly-once delivery over an unreliable network is impossible, so systems achieve exactly-once EFFECTS by pairing at-least-once delivery with idempotent processing (dedupe on a message id) or transactional writes. Gotcha: when a vendor advertises “exactly-once,” it usually means exactly-once processing within their boundary under specific conditions — cross that boundary and you are back to designing idempotent consumers yourself.