Alright kiddo, so you know how when you do something really nice or really good, people around you will say "Wow, that's really great!" And sometimes they might even say "That's the right thing to do!" Well, when Jewish people do something really good or really nice, and it makes other people around them say those kinds of things, that's called "kiddush hashem."
Basically, when Jewish people make a kiddush hashem, they're doing something that makes people see the good in Judaism and in Jewish people. It could be something as simple as a kind gesture, or something bigger like standing up for what's right. And when they do these things, it makes other people think, "Wow, Jewish people are really good and kind!" And that's a really great thing for everyone, because it helps make the world a better place.
So, to sum it up, kiddush hashem is when Jewish people do something good or kind that makes other people see the good in Judaism and Jewish people. It's like showing the world that being Jewish means being a really good person!