A Hard Fork (sometimes also known as Hardfork) is a change in the protocol that helps in making invalid transaction valid. It requires users or nodes in order to upgrade the protocol software latest version. In the other words, a hard fork is totally different from the previous version of nodes and blockchain that are not accepted by the latest version. It can be implemented for security and it is not found in the previous version of the software. Hard Fork will add new functionality and reverse transactions in Ethereum’s blockchain.
The hard fork consists of the splitting of blockchain through making the transactions invalid that are nodes confirmed and not upgraded to protocol software latest version. The Ethereum community has voted in the favor of Hard Fork so that it can roll back the transactions of around millions of Dollars of cryptocurrency done by a hacker. In addition, Hard Fork has also permitted DAO token holders for getting Ether funds back.
In simple words, you can explain Hard Fork as any change in the block structure, rules, and set of confirmed transactions of Bitcoin. These changes can easily be implemented by updating old clients about the new transaction appear.