Our universe is very, very, very big and the common units of distance measurement (such as meter, kilometer, miles, etc.) will simply not suffice for measuring distances in our vast universe. For example, our nearest star, Sun, is about 149,600,000 km from us. The next nearest star, Proxima Centauri is about 40,140,000,000,000 km away from us. And there are about a billion trillion stars in our Universe and hence it would be extremely difficult to deal with such distance based on our conventional units.
A very commonly used unit for measuring such large distances is light years or the amount of distance traveled by light in one year. Since light travels at about 300,000 km/s, a light year is equal to about 9,500,000,000,000 km. This unit is sufficiently large to report distances in our Universe. For example, Proxima Centauri is about 4.24 light years away from us. This distance, in terms of light years, is much easier to report and write, than in kilometer or other commonly used units in our daily lives.
Hope this helps.