This tag should be used for questions regarding how Proof of Work works and Proof of Work algorithms. Proof of Work is a scheme where a 3rd party can verify that someone performed at least a certain amount of work to produce something. This is used in Bitcoin mining.
The Proof of Work scheme is one where one can verify that another person performed, on average, a certain amount of work. It is easy to verify a Proof of Work but hard to produce one.
Bitcoin uses Proof of Work in mining. Miners continuously modify and hash blocks in order to find a hash which is less than the target. This process is difficult and requires a lot of work, but is easy for everyone else to verify and prove that the hash is indeed less than the target. Because the target is a specifically chosen small value, having a hash that is less than that target means that the miner must have done a certain amount of hashes, on average, to produce the block hash.