SHA-256 stands for secure hash algorithm. SHA-256 transforms a key or an input into a hash with an output of the size 256 bits. The input key can have any possible length, but the hash will always have 256 bits. The hash function is deterministic, meaning that the same input always gets the same output. Moreover, the hash algorithm is designed so that from the input the output can be determined, but from the output the input cannot be determined. The only way to see what a hash had as an input is to brute force the algorithm. This means that a computer must try out every possible combination for the key to find the matching hash. This seems to be a very hard task, which is technically possible but in practice it is not because it would take decades to achieve.