Well kiddo, there's something called presentism, which is a fancy word for the belief that only the present moment exists. Some people who believe in presentism think that the past and future aren't real, they're just ideas in our heads.
Let me give you an example. Imagine you had a birthday party last year. If you're a presentist, you'd say that the birthday party isn't really happening anymore. It's only in your memory. The same goes for tomorrow. Even though you might have plans for tomorrow, it isn't really happening yet.
Now, some grown-ups believe that presentism is true, while others don't. Those who don't believe in presentism think that the past and future are just as real as the present.
It's a lot to think about, but the important thing to remember is that people have different ideas about what is real and what isn't. And that's okay! It's good to ask questions and try to understand different perspectives.