A consensus mechanism is a method used by a blockchain network to validate the authenticity of transactions. It is also the process used to keep the network secure. The consensus mechanism confirms that all transactions within the blockchain get recorded and that all blocks contain valid information about the transactions.
Think of the consensus mechanism as the procedure a blockchain must follow to validate every transaction taking place on the distributed ledger technology before recording it. Popular consensus mechanisms include Proof of Work (Pow), Proof of Stake (PoS), Proof of Authority (PoA), Delegated Proof of Stake (DPos), and Proof of History (PoH).