When people tell stories, they sometimes use special ways of talking that make the story more interesting. One way of doing this is called "climax."
Imagine you are telling your friend about a really exciting movie you watched. You could just say, "The movie was exciting." But if you want to use climax, you might say something like this: "First, the bad guys were chasing the superheroes. Then, the superheroes were fighting against a monster. Finally, the superheroes had to save the entire city from exploding! It was SO exciting!"
In this example, the climax is the most exciting part of the story - when the superheroes have to save the city from exploding. By starting with less exciting events and building up to the climax, the storyteller makes the climax even more exciting.
People use climax in all sorts of stories: books, movies, even speeches! It's a way of making the audience more interested in what's happening, and keeping them on the edge of their seat until the very end.