The significance of a blockchain consensus mechanism lies in its role in achieving agreement among network participants on the validity of transactions within a blockchain. This mechanism ensures that all parties in the network adhere to a specific protocol for verifying and recording transactions. It enhances security and trust by preventing fraud, double-spending, and unauthorized alterations. Different consensus methods, like Proof of Work and Proof of Stake, offer various trade-offs in terms of speed, energy consumption, and decentralization.