The grandfather paradox or consistency paradox happens when the past is manipulated in any way, thus resulting in a contradiction.
Let’s suppose you have a time machine that allows you to travel back into the past. While you’re there, you accidentally kill one of your grandparents before they have any offspring. This would lead to a situation where your parent (grandfather’s child) would not be born at all. As a result, you also wouldn’t be born at all. But if you weren’t born in the future, then you couldn’t kill your grandfather in the past. So, who killed the grandfather in the first place?
The grandfather paradox is a self-contradictory situation that arises in some time travel scenarios whenever changing the past is viable.