A light year is the distance that light travels in a year. The speed of light in vacum is 299,792,458 meters per second, and there are 31,556,926 seconds in a year. Therefore, a light year = 299,792,458 meters per second * 31,556,926 seconds = 9.4605284 × 10¹⁵ metres.
A light year is just under 10 trillion kilometers. The use of light year as a measuring unit is easier for astronomers to indicate the distance in the universe.
The nearest star to the sun - Proxima Centauri, is about 4.2 light years away from earth, i.e. it takes light travels 4.2 years to reach earth. Can you image if we write that in kilometers? It will be about 42 trillion kilometers. The width of the Milky Way Galaxy is about 150,000 light years.