what is "Herd Immunity"?

Had a friend annoyed at masks the other day, complaining heavily "muh human right to shop at the supermarket without wearing a mask!" Interlocutor: So you'd prefer herd immunity? Complainer: Of course! Weren't you even listening?! I: Well, do you know what herd immunity means? What is it? C: Ah, may be not? What is herd immunity? I: If you survive, you survive; those who die, are left to die, the rest of the herd is immune… C: <silence> I: So you've heard a little about the Spanish flu and the Black Death, right? C: Yeah.. I: Well, we're the survivors - we are that part of the herd that did not die.