One-way functions are those functions that can only practically be computed one way; that is to say, they are easy to compute one way and 'hard' to compute in reverse. In this case, easy and hard refer to computational complexity or the amount of computer resources required to run an algorithm. Generally, one-way functions refer to functions that can be completed one direction under polynomial time complexity (P) but follow non-polynomial time complexity (NP), often NP-hard, in reverse.
These functions are especially useful for a wide range of applications in cryptography, from passing messages to storing passwords, and are fundamental to the essence and security of the modern TCP/IP internet stack. Within the context of blockchains, many hash functions are 'one-way'.