Imagine this, you just watched a movie and you absolutely loved it. You want to tell all your friends that they should not miss it. You can say, “The movie is so terrific and unmissable.”
Basically when you say something is unmissable, you’re saying “it’s awesome” and “don’t miss it“.

Unmissable things need not be just movies and TV Shows, it can be a huge discount sale which you don’t want your friends to miss, a cute expression of a baby when angry.