A hard fork is basically a change in protocol that is not backwards compatible. Its real important for bitcoiners and the bitcoin eco system, that they don't do any hard forks and if they do decide to, it turns into a completely separate chain like bitcoin cash (BCH) or bitcoin Satoshi vision (BSV). Other protocols it's less important to them, it's more like a move fast, break things, and correct. Ethereum and other protocols hard fork all the time. Some people tend to think it's cleaner, better, and leads to progress. Keep in mind that it's very important for bitcoin not to fork and to stick with its original simplistic fundamentals, just as Satoshi designed it, with complete decentralization and unbreakable proof of work. Kind of like the constitution of the United States, not to be changed! Bitcoin is like the constitution of crypto. Anybody new to the market doesn't quite understand, and there use to be a lot of confusing over this in 2016 and 2017, there was what's known as the Fork Wars. There was uncertainty in the market; with what gives bitcoin value, is bitcoin the valuable one, or are all these clones like Litecoin (LTC) or forks like BCH and BSV a better bitcoin. Over time the community chose Bitcion, which was the right move, because they prioritized censorship resistance, better security, and a few more fundamental characteristics. Bitcoin doesn't get 51% attacked, its proved its strength time and time again through various hard ship.