None of these are ELI5 so I’ll have a go.
Your friend throws you a ball every second. You catch it every second.
But now they start to walk backwards away from you. They are throwing a ball every second but because they are moving away, you don’t catch one every second any more, it’s one every two seconds.
The same thing is happening with light from stars. Light that arrives less often than it was sent out looks to us more red than it would if the object wasn’t moving away.
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