When we talk, we use different "cases" in our sentence to show who is doing the action (subject), who or what is receiving the action (object), and other information.
One of these cases is called "Dative," which is used to show the person or thing that is indirectly affected by the action. For example, "I gave the ball to my friend" - "my friend" is the indirect object because they receive the ball, but not directly.
Another case is called "Genitive," which is used to show possession or ownership. For example, "That is my grandmother's house" - "grandmother's" shows that the house belongs to her.
However, in the German language, people sometimes use the Dative case instead of the Genitive case, which can create confusion or sound strange to people who are used to the Genitive case.
So, "Der Dativ ist dem Genitiv sein Tod" is a phrase that means "The Dative is the death of the Genitive." It's basically saying that people are not using the Genitive case correctly or often enough, and instead using Dative incorrectly in its place.
It's a controversial topic in the German language world, and some people feel very strongly about it (just like adults sometimes feel strongly about different opinions!). But for now, just remember that sometimes people use the wrong case and it can cause confusion.