Okay kiddo, Hebrews is a book in the Bible that tells us about what Jesus did for us. Chapter 10 talks about how Jesus sacrificed himself for us and how we can't just keep doing bad things and asking for forgiveness over and over.
It's like if you're playing with your toys and you accidentally hit your friend. You say sorry and they forgive you, but then you do it again and again. Eventually, your friend might not forgive you anymore because they're tired of you doing the same thing over and over.
That's why the writer of Hebrews tells us that Jesus' sacrifice was a one-time thing, and we shouldn't keep doing bad things and asking for forgiveness over and over. Instead, we should try to do better and live in a way that honors Jesus' sacrifice.
The writer also encourages us to keep meeting together and encouraging each other to do good. It's like when you have friends over to play and you all help each other, share your toys, and have fun together. In the same way, we should help each other in our faith and encourage each other to do what's right.