It is a system that requires a practical yet significant amount of effort to prevent malicious and pointless use of computing power, like sending spam emails.
The Long Explanation
Originally proposed as an anti-spam feature to prevent denial-of-service attacks (DoS), Proof of work (PoW) is now used as a security system for cryptocurrencies, including Ethereum, to oversee transactions.
To keep financial transactions secure, the Proof of Work (PoW) system makes people, called “miners”, solve complex puzzles using energy-consuming machines. This then allows them to add a verified transaction block to the blockchain, for which they receive a predetermined transaction fee or new tokens as a reward.
