The number of confirmations indicate how deep your transaction is in the blockchain. When a transaction is confirmed once, it means that the transaction is included in the last block that has been added to the blockchain. By waiting for a transaction to be included in a block, you can prevent bitcoins or other cryptocurrencies from being double spend or for transactions to be reversed. Waiting for confirmation simply offers more certainty. The more confirmations a transaction has, the deeper in the blockchain and the more secure your transaction is. Rolling back a transaction is very unlikely after 3 confirmations and virtually impossible after 6 confirmations.
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