When we look up at the night sky, we see lots of little lights. The lights are from stars like our sun that are really far away. However, their light still reaches us in time. If one of those stars exploded in a supernova, we wouldn't know for a long time because it takes time for the light of the explosion to reach us. Lets look at the example below.
So as you can see, many of the stars in the night sky may not even exist anymore. We just don't know because the light hasn't stopped coming because of how long of a trip the light has to take.