The process on which PoW consensus is based upon. Miners use computational power through cryptographic hash functions to secure the network.
How hash functions work
Mining is based on cryptographic hash functions. A hash function is a special kind of cryptographic process that generates a fixed amount of bits given an arbitrary input. An example of an hash function is SHA-256 (which is used to secure the Bitcoin blockchain or Keccak-256 (used in Ethereum).
Hash function are one-way functions, this means that it is easy to derive the output given the input, but it is computationally infeasible to do the opposite; this means that one cannot retrieve the original input given the output of the hash function. Also, for any given input there is a unique output such that, even if only 1 bit of the input is altered, the output changes completely and is completely unrelated from the previous one.