Short answer: A hard fork is when a group of people decide they no longer like the direction a crypto is going (or can't get to an agreement) and announce to the world that, at some specific moment in the future, they'll continue building the chain with their rules/technology.

This creates 2 chains with identical history up to the moment of the split (aka: hard fork).

Chain1: A -> B -> C -> D ....

Chain2: A -> B -> C-> X -> Y ...

Usually the chain with the most support gets to keep their name and the "loser" changes theirs.

Happened with Ethereum, but also Bitcoin, and probably others.

In the screenshot the dude is asking his gpt bot to look up what specific block was the last one to be in the shared history before a split.