Ok, let's imagine that you and I are playing a game of "I Spy" and you say, "I spy something green." But instead of telling me what that green thing is, you stay quiet and don't say anything. That's called an "argument from silence."
Sometimes when people are trying to prove something, they might use an argument from silence by saying that something is true just because nobody has said it's not true.
But just like in our game of "I Spy," when someone doesn't say something, it doesn't mean it's not there. So we have to be careful when someone uses an argument from silence to make sure that they're trying to prove something with real evidence and not just because nobody has said anything against it.