A light-year is a unit of measurement used to express astronomical distances. It is the distance that light can travel in one year, which is approximately 5.88 trillion miles (9.46 trillion kilometers).
Since light is one of the fastest things in the universe and travels at a speed of approximately 186,282 miles per second (299,792 kilometers per second) in a vacuum, it can cover a vast distance in a relatively short amount of time. This means that even relatively nearby stars and galaxies can be measured in light-years, due to the vast distances involved in space.
The concept of a light-year is used to express the distance between objects in space, such as stars and galaxies. For example, the nearest star to Earth, Proxima Centauri, is about 4.22 light-years away, while the Andromeda galaxy, which is one of the closest galaxies to the Milky Way, is approximately 2.5 million light-years away.
It's also important to note that a light-year is a measure of distance, not time. When we say that a certain object is X light-years away, it means that the light we are seeing from that object today left that object X years ago.
In addition to light-years, other units of measurements such as parsec (pc) and astronomical unit (AU) are also used to express astronomical distances.