Candaulism is when someone gets excited by watching their partner have sex with someone else. It's like when you play with your toys and you like to share them with your friends because it makes you happy to see them having fun too.
But for some adults, they like to share their partners with other people in a sexual way. They enjoy watching their partners have sex with others because it makes them feel good to watch their partner having fun.
It's important to remember that candaulism is only okay if all the people involved are happy and excited about it too. Just like how it's important to ask your friends if they want to play with your toys before sharing them, adults need to communicate and make sure that sharing their partners is something that everyone is comfortable with.