When we look at the stars, we are seeing them as they were years ago. If a star is 100 light years away, we are seeing it as it was 100 years ago, etc. If a star dies (goes nova), we won't know about it for the year-equivalent of its distance in light-years. Then, one day, we'll see a flash in the sky and we'll know that if it is 27 light-years away, the star died 27 years ago.