You probably know that light has a speed, right? It takes time for light to get from point A to point B. Even from our Sun, light takes around 8 minutes to reach us. From that, you are actually viewing the Sun (relative to yourself) from about 8 minutes ago, by the time the light reaches you. If the Sun just disappeared magically, you wouldn't really know it for about 8 minutes while the light traveled from where the Sun used to be, to your eyes on Earth. So, expand that concept to distances that are billions of not just minutes, but years. A "light year" is just how far light travels in an Earth year. So, we can see about as "far" as 13.5 billion light years away. Which also implies we can see as "far back" (in time) as 13.5 billion years ago.