Like people and cars, light has a speed, so even though it moves very fast it still takes time to arrive at its destination. When you get a postcard from a friend on vacation, you learn what they were doing last week when they sent it, not what they're doing the moment you read it. When you look at a planet, star, or galaxy, you're also receiving a message from the past, a message that tells you what the bright object was doing at the moment it sent the light to you. When you look at the sun (indirectly, of course), you're seeing the light it emitted eight minutes ago. Jupiter appears as it was at least half an hour ago.