Ok kiddo, have you ever seen a really cool game or concert on TV or in person? The people who made the game or concert want everyone to know about it and they try to sell tickets or merchandise to make money. But sometimes, there are other companies who didn't make the game or concert but they still want to make money from it too. That's called ambush marketing.
It's like if you were having a lemonade stand and you worked really hard to make the lemonade and painted a sign to tell people about it. Then, your neighbor comes outside with a different sign saying he also has lemonade for sale but he didn't even make any! That wouldn't be very nice, would it?
So, ambush marketing is when a company tries to make money off of something that they didn't really have anything to do with, without asking for permission or paying anyone for it. This happens a lot during big sports events like the Olympics or the World Cup when companies will make ads or commercials that make it look like they are part of the event, when really they aren't.
Some people think ambush marketing is unfair because the companies who actually made the event or the products won't get all the money they should. It's like someone trying to take credit for something they didn't really do!