Hey kiddo, have you ever played dress-up and pretended to be someone else? Well, that's kind of like what passing off means. It's when someone tries to make someone else think they are something they're not.
Let's say you have a friend who always gets the newest toys and you really want to have one too. But you don't have enough money to buy it. So, you decide to borrow your friend's toy and pretend that it's yours when you're around other people. That's passing off - you're pretending to be something you're not.
Passing off is usually a bad thing to do. It can hurt other people's feelings and it's not very honest. Just like how you wouldn't like it if someone pretended to be you, it's important to be truthful and only pretend in playtime when everyone knows it's just for fun.