Proof of Work is the consensus protocol used in Bitcoin, and other cryptocurrency platforms, to verify information (confirmation). Under this system, miners compete to solve difficult mathematical puzzles to verify information. The winners get to add the blocks to the blockchain. This process requires significant computer power so miners receive new coins as a reward. The Proof of Work protocol makes transactions more secure by significantly reducing the risk of double spending.
Proof of Work protocols are not 100% secure. While difficult, a hacker could disrupt a system if they took control of at least 51% of the network. This is called a 51% attack.