That means it’s not a 100% instant response as some people are sleeping (can’t reply at the moment) but reply when you’re asleep.

If you go outside you’ll notice there’s this thing called the sky. It gets dark sometimes. Sometimes its bright. That’s because of the sun. People like to sleep when it is dark. When the sun is on the other side of the world its day time for them. But night time for you. So you are sleeping and they are awake. Even if we just used universal time for everyone, the sun would still do the same thing. And people would still be asleep while you are awake.