A light year is about 5.88 trillion miles - the distance light travels in one year. It's used because regular miles would be ridiculous when talking about astronomical distances. The nearest star besides our sun is about 4.24 light-years away.
So How Fast Is Light?
Light travels at about 186,000 miles per second. In one second, it could circle Earth about seven times. In one year, it travels about 5.88 trillion miles. That's fast!