Hard fork
In short A hard fork is a blockchain rule change that old software rejects, so every node must upgrade or end up following a separate chain.
A hard fork is a change to a blockchain's protocol that is not backward compatible. Blocks valid under the new rules are rejected by nodes still running the old software. If everyone upgrades, the network simply moves to the new rules. If a meaningful group refuses, the chain splits into two networks that share history up to the fork block.
Most hard forks are planned upgrades. Ethereum has used them many times to change fees, add features, and move to proof of stake. Contested forks are different: in 2016, after the DAO hack, Ethereum hard forked to return stolen funds, and the minority that kept the original chain became Ethereum Classic. In 2017, a dispute over block size led to Bitcoin Cash splitting from Bitcoin. A soft fork, by contrast, tightens rules in a way older nodes still accept.
For holders, a planned upgrade usually requires no action if your coins sit in a self-custody wallet, since your keys work on the upgraded chain. Exchanges may pause deposits and withdrawals around the fork. In a chain split, you typically end up with coins on both chains, but their market value and exchange support are not certain, and tokens issued on top of the chain may only work on one side.
Watch out for scams around forks. Fake wallets and websites ask for your seed phrase to claim forked coins; never enter it anywhere except your own wallet. During a split, wait for replay protection to be confirmed before moving funds, so a transaction on one chain cannot be copied onto the other. Check official announcements from the chain's core developers and your exchange instead of social media posts.