A light year is a unit of distance that measures how far light travels in one year. Since light moves at an incredible speed of about 299,792 kilometers per second (or about 186,282 miles per second), a light year equals approximately 9.46 trillion kilometers (or about 5.88 trillion miles). This vast distance helps astronomers describe the immense spaces between stars and galaxies. When we say a star is, for example, 4 light years away, it means that the light we see from that star today actually left it 4 years ago. This concept is crucial for understanding the scale of the universe, including objects like the Milky Way and Andromeda Galaxy.