Hard fork is defined as a disagreement in the operations of a cryptocurrency followed by a decision to make a permanent change to the technology resulting in part of the group splitting off to form their own cryptocurrency.
This change makes all new recordings (blocks) very different from the original blocks. They are changed so much that new blocks are seen as invalid to anyone who didn’t upgrade their technology. Which means, any computer that is not updated with the new technology, will find these new blocks appear invalid.
This process can cause a lot of trouble which is why, for a hard fork to go smoothly, it is important everyone agrees to the change. Any new fork in the blockchain can fail and if it does, all users will return to the original recording.