A Hard Fork (or hardfork), as it relates to blockchain technology, is a radical change to a network's protocol that makes previously invalid blocks and transactions valid, or vice-versa. This can be done for a variety of reasons, such as fixing security issues or implementing new features.Hard forks can be contentious if not everyone agrees on the changes being made, which can lead to two separate chains emerging from the original one. In order for everyone to agree on which chain is the "true" one, usually a majority consensus is required. However, this isn't always possible and sometimes there are irreconcilable differences between groups of people who want different things from a blockchain network.
