Okay kiddo, so imagine you want something really cool for your birthday - like a big toy, but you also want some colorful balloons, and maybe even a party hat. All of these things are what we call decorations or ornaments.
Now, imagine there are some animals out there that want to look really good to attract a mate - kind of like trying to impress a best friend at a party. They might have different things they use to decorate themselves with, just like we use different things to decorate our birthday parties.
Some animals have really bright feathers or fur, like a peacock with its big, colorful tail feathers. This is an example of a sexual ornament, which is something an animal has that helps them attract a mate.
Now some animals have more than one sexual ornament - these are called multiple sexual ornaments. For example, some birds have bright feathers, fancy dances, and even sing really loudly to try and impress a mate. It's like they're using a whole bunch of decorations to make themselves stand out even more.
So just like you might use multiple decorations to make your birthday party super cool, animals might use multiple sexual ornaments to make themselves super attractive to a mate.