Why is the distance between stars expressed in light years? What do you understand by the statement that a star is 4.3 light years away from the earth?

This is because the distance is extremely long and lightyear is a very big unit of distance. 4.3 light years means that light will take 4.3 years to cover the distance between the stars. This means that if an event happens on one star, you will be able to look at it after 4.3 years from the other star.