PoW is the backbone of Bitcoin: miners expend computational resources to produce valid blocks. They combine a block’s data with a nonce, repeatedly hashing until the output is below the difficulty threshold. The ‘work’ is verifiable by any node but extremely costly to replicate at scale, deterring double-spends or chain rewrites. PoW secures the blockchain without a central authority: to attack, one must outcompete honest miners in raw hashing power. Critics cite high energy use, while supporters argue it underpins Bitcoin’s robust, permissionless security.