A light year is a unit of distance that measures how far light travels in one year. Since light moves incredibly fast—about 186,282 miles per second (299,792 kilometers per second)—a light year equals about 5.88 trillion miles (9.46 trillion kilometers). This measurement helps astronomers understand the vast distances between stars and galaxies in the universe. For example, when we say that a star is 4 light years away, it means that the light we see from that star today actually left it 4 years ago. This concept is essential for studying celestial objects like stars, galaxies, and even black holes, giving us a better understanding of the universe's structure and history.