The law of return is a belief in Judaism, which says that whatever you do, good or bad, you will receive the same in return. So, if you do something good, like helping someone in need, you will receive good things in return, like happiness or blessings. But, if you do something bad, like hurting someone or telling lies, then bad things can happen to you, like getting in trouble or feeling sad.
It's kind of like a rule that says if you plant a seed, you will eventually get a plant. If you plant a good seed, like a tomato seed, then you will get good tomatoes. But if you plant a bad seed, like a weed seed, you will get a bad weed. Similarly, if you do something good, you will get good things in return, and if you do something bad, you will get bad things in return.
Now, this rule doesn't mean that you will get the same thing in return for everything you do. For example, if you smile at someone, you might not get a smile back, but you will feel good inside. And, if you hurt someone, you might not get hurt in the same way back, but you might feel bad inside.
Overall, the law of return is a reminder for us to be kind and do good things, and to avoid doing bad things because they can come back to us in some way or another.