Okay kiddo, imagine you learn to ride a bike. You might practice every day and get really good at it, but if you stop riding your bike for a few months or a year, you might not be as good as you once were. This is kind of like second-language attrition.
Second-language attrition is when you learn a second language, but then you don't use it as often, so you kind of forget how to speak it. Just like how you might forget how to ride a bike if you don't ride it for a long time.
When you learn a new language, your brain makes new connections and pathways to help you remember and use the language. But if you stop using the language, those pathways might not be as strong anymore, kind of like a muscle that hasn't been used in a while. So, just like how you need to keep practicing riding a bike to stay good at it, you need to keep practicing speaking a second language to stay good at it.
This can happen to anyone who speaks more than one language, even adults. But it's especially important for kids because they're still learning and growing their language skills. That's why it's important to keep using your second language, even if you don't use it every day.