Imagine a highway with clear rules for which lane cars can be in. A hard fork is a major change to those rules. It's like suddenly saying trucks can now use the carpool lane, and cars are no longer allowed on the highway at all! This creates a permanent split, where the old way (invalid transactions become valid) and the new way (valid transactions become invalid) can't coexist. Hard forks are risky and not very common because they can cause confusion and instability in a blockchain network.
