A light year is a unit of distance used in astronomy. It's the distance light travels in a year. It is not in any way a unit of time, though it sounds like it.
The light year is usually used when expressing the distance between two astronomical objects, such as stars. A light year measures how far light travels in the Julian calendar year, or 365.25 days. Light travels at 299,792,458 meters per second. This means that light travels 9,460,528,400,000,000 meters in a year. So, a light year is equivalent to 9,460,528,400,000,000 meters. A single light year is so large it can circle around Earth about 236,070,578 times.