A hardfork is a permanent deviation from the previous version of the blockchain. Nodes that run on earlier versions will no longer be accepted by the latest version. This essentially creates a fork in the blockchain: one path follows the new, updated blockchain, the other path follows the old blockchain.
For example, a hardfork can be implemented to correct important security risks found in older versions of the software, add new features, or reverse transactions. A prominent example of this is the Ethereum hardfork which was performed due to the “DAO Hack”.