How are light years used to measure distances in the universe

A light year is a unit of distance. It is a distance that light can travel in a years time which is six trillion miles. It is used to measure the distances in space. To take one example, the distance to the next nearest big galaxy, the Andromeda Galaxy, from earth is 21,000,000,000,000,000,000 km. 
Do you understand it? 

It equals just about 6,300 km per hour each day an week