Proof of Work is a blockchain consensus method. It requires miners to use computing power to solve difficult problems before they can add a new block.
Why Proof of Work is secure
Because mining costs electricity and hardware, attacking the network becomes expensive. To rewrite history, an attacker would need enormous computing power.
Where it is used
Bitcoin is the most famous proof-of-work network. Some other cryptocurrencies also use it, while many newer networks use proof of stake.
Proof of Work is respected for security and simplicity, but it is often criticized for energy use and hardware concentration.