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

Hint Distance between two is very large as compared to a distance of one kilo meter that is why we need a unit whose unit value is too much larger in comparison to one kilometer. A light year distance is very large and it is equal to the distance traveled with the speed of light for one year.

Complete step-by-step answer:
The distance between two stars is expressed in units of light years because the distance between two stars is a very large distance and we need a unit which can be very large and we calculate distance in terms of this unit easily. A unit light year is distance covered by an object moving with speed of light for one year.
We know the speed of light is $3 \times {10^8}m/s$ and with this speed an object covers a distance of one light year or we can say that light takes one year to cover a distance of one light year.
A star is eight light years away from earth means that light coming from that star takes eight years to come from that star to earth.

Note A light year is a unit of distance used to measure distance between two stars. If we see a star from earth which is eight light year away from earth then the light we see coming from that star leaves the star eight year ago.