A light year is essentially the distance that light can travel in one year. One year = 31,536,000 seconds. Light covers a distance of 300,000 kilometers a second. Therefore, in one year light is capable of traveling 9,460,800,000,000 kilometers.
The nebula closest to Earth, the Orion nebula, is 1,300 light years from us and stretches to a distance of 25 light years. (factslegend.org)