Like other electromagnetic particles, light has a specific velocity at which it travels. Further, it is presumed to be the fastest theoretical speed of any particle. The speed of light–referred to as “c” in a vacuum–is roughly 300,000,000 m/s. Since it moves so fast, most people never observe light as a moving particle or something that takes time to see. However, in the scale of the universe, the constant “c” can be useful to describe fast distances.
A light year is the distance that light travels in a year at its constant speed “c.” This is not only useful for breaking down/assigning meaning to vast distances, but it is also useful to think about in terms of our observation of the universe. If something is “x” light years away, the light we observe from it on earth is “x” years old. Essentially, we are looking into the past.
Here is a diagram putting the speed of light into context with our solar system and galaxy. It is taken from a Quora post.
