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The mixin count refers to the number of other signatures (aside from yours) in the ring-signature that authorizes the transaction. So a mixin of 3 means that there are a total of 4 signatures. This means that someone looking at the monero blockchain has no way of knowing which of the four signers is the true sender.[1]
  1. https://bitcointalk.org/index.php?topic=753252.msg9985441#msg9985441