Proof-of-Work — an algorithm to protect distributed systems from malicious activity (spam-mailing, DoS-attacks).
The computer solves PoW tasks since their solution requires a vast amount of computing power. And a key feature is the asymmetry of time costs: verification of the solution requires much less time and resources than it takes to locate it.
The most popular and developed PoW-technology is in cryptocurrency systems. For example, in the Bitcoin network, PoW was used as a means of reaching consensus — a common opinion on which version of the blockchain is considered correct.