Okay kiddo, so have you ever tried to do something that was too hard for you? Like carrying a really heavy box that you couldn't manage? That's kinda like what it means to be "in over our heads". It means we've taken on a task or a problem that is really difficult or complicated and we don't know how to handle it.
Sometimes, grown-ups can get in over their heads with things like managing money, or trying to fix things that are broken. It can be really scary and overwhelming, like we're drowning in the situation. When we're in over our heads, it's important to ask for help so we don't make things worse.
So, just like when you need help reaching something high up, or tying your shoelaces, grown-ups sometimes need help too. It's okay to admit when we don't know how to do something, and to ask for someone else's knowledge or expertise to help us out. That way, we can learn and grow and not stay in over our heads for too long.