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A host may send multiple packets in quick succession, one or more of these packets may be lost. Upon receiving a packet, the receiver will reply with an ACK. If the sequence number of the incoming packet is not what the receiver expected, it will reply with the same ACK as the last packet which was correctly in sequence.
If the sender receives three duplicate acknowledgements in a row, it will retransmit from the smallest sequence number which has not yet been ACKed.
This might all occur before a timeout occurs for the lost packet. The receipt of three duplicate ACKs indicates to the sender that a packet has been lost, and it does not need to wait for the timeout to confirm loss.