Okay kiddo, sometimes people are born and they don't feel like they fit into just being a boy or just being a girl. They might feel like they're both, they might feel like they're neither, or they might feel like something completely different.
But some people don't understand or accept this and they might treat these people unfairly, or discriminate against them. This means they might not give them the same opportunities or treat them the same as they would someone who identifies as just a boy or just a girl.
It's like when some kids at school only want to play with certain types of toys, and they make fun of the other kids who want to play with different toys. It's not fair or kind, and we should always try to treat everyone with respect and acceptance, no matter how they identify or what toys they like to play with.