Okay kiddo, let's talk about mortal sin.
You know how we have rules we need to follow, right? Like we can't hit our friends or take toys that don't belong to us? Those are kinda like God's rules, or what we call "commandments".
Mortal sin happens when we break one of God's rules in a really, really big way. Like if we hit someone really hard and hurt them on purpose, or if we steal something really valuable.
But it's not just breaking the rule that makes it a mortal sin - we also have to know that it's a really bad thing to do, and do it anyway. It's kinda like if we know we're not supposed to touch a hot stove, but we do it anyway even though we'll get hurt.
So mortal sin is breaking a really important rule on purpose, even though we know it's a really bad thing to do. And when we do that, it makes God really sad and it hurts our relationship with Him.
But the good news is, we can always say sorry and ask God for forgiveness. He loves us no matter what, and He always wants us to ask for help if we're having a hard time following the rules.