A dead letter queue is a queue that is used to hold messages that cannot be processed successfully. This might happen if the message is malformed, or if the message fails to be processed after a certain number of attempts. Dead letter queues can be a useful tool for debugging purposes, as they can help you to identify why certain messages are not being processed correctly.

https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-dead-letter-queues.html