The key concept to understand here is that the speed of light is finite, and light takes time to travel across vast distances in the universe. When we observe distant objects like galaxies, we are essentially seeing them as they were in the past because their light takes time to reach us.
GN-z11, currently the most distant known galaxy, is estimated to be about 13.4 billion light-years away from Earth. This means that the light we receive from GN-z11 today has traveled for approximately 13.4 billion years to reach us.
To clarify, the term "light-year" is a unit of distance, not time. It represents the distance that light travels in one year, which is about 9.46 trillion kilometers or 5.88 trillion miles. So, when we say GN-z11 is 13.4 billion light-years away, we are describing the distance that light has traveled to reach us, not the time it took for that light to travel.
Since the speed of light is constant, it allows us to make these calculations and estimate the age of the universe. When we observe objects that are billions of light-years away, we are effectively looking back in time, seeing them as they existed billions of years ago due to the time it took for their light to reach us. This understanding helps us unravel the history and evolution of the universe.