Have you ever heard of a unicorn? It's a magical creature with a horn on its head, sort of like a horse. Sometimes, people use the word "unicorn" to talk about things that are really special or rare.
Now, imagine that some grown-ups want to find a unicorn. But they're not looking for the kind with the horn. They're looking for a special kind of person who they think is really rare and special. They call this person a "unicorn."
They might say they want to find a "bisexual woman who is interested in having a threesome with a couple." That means they want to find a woman who likes both boys and girls AND doesn't mind having sex with a couple who are already in a relationship.
Some people think that it's not very nice to go looking for a person like this. They might feel like they are treating the "unicorn" like an object to be hunted or used rather than a real person with feelings.
So, unicorn hunting basically means trying to find a specific type of person to join in on sex or a relationship, and some people don't think it's a nice thing to do.