The dictionary defines light year as the distance that light travels in a vacuum in 1 year which equals 5.88 trillion miles. A light year is a measurement of distance, not of time. Light can be used to measure distance since it is the fastest known thing in the Universe.

It takes light about from the sun about 5.5 hours to travel across the solar system. The nearest star to Earth is about 4 light years away. The Milky Way Galaxy is a disc about 100,000 light years long and around 2000 light years wide. That means, even if you could travel at the speed of light, it would take you 100,000 years to travel across it.

Light speeds along at 186,000 miles per second or about 670 million miles per hour. At that rate, light will travel almost 6 trillion miles in a year. That means that when you look at a star that is 100 light years away, you're seeing the light that star gave off 100 years ago. In fact, many of the stars you look at in the sky at night, may no longer exist.

Human do not currently have technology capable of traveling anywhere near that fast. In fact, Albert Einstein, in his theory of relativity said that it would be impossible to accomplish.