A Bitcoin hard fork can be thought of as a literal “fork in the road”.
The blockchain does its job, processing transactions according to its protocol when a split occurs and there are two different chains, the new and the old. This split is known as a hard fork and often results from disagreements over how the original protocol works.
There’s been a lot of these disputes over the history of bitcoin, but the new chains haven’t caught on and the free market has decided that the unchanged bitcoin protocol is the main one they want – it looks like they’ve chosen wisely if you ask us!