After a transaction has been included in block n, it has a confirmation. As soon as a new n+1 block is released and distributed in the network, the transaction has two confirmations. Each additional block linked to the Blockchain adds one confirmation to the transaction. It is considered that 6 or more confirmations are sufficient to ensure the non-reversibility of the transaction. It should be noted that 6 confirmations are acceptable for the bitcoin blockchain. For the other blockchains, it’s different.