Proof-of-work (PoW) is a consensus mechanism that regulates the process of adding blocks of transactions to the blockchain. Bitcoin is one of the few cryptos using this system. Proof-of-work has excellent security and is more decentralized compared to Proof-of-Stake (PoS) systems. However, it is also inefficient (hard to scale) and environmentally unfriendly.
In the PoW system, miners verify and process all transactions. Each miner competes with each other to solve cryptographic puzzles and upload blocks to the blockchain. The winning miners will get Bitcoins as a reward.
Furthermore, the process of solving this cryptographic puzzle is very difficult and requires special computers called ASICs (Application Specific Integrated Circuits).