Okay kiddo, so you know how boys and girls have different parts on their bodies, right? Well, some people are born with a mix of these parts or parts that don't clearly fit into "boy" or "girl," and this is called intersex. But, there are also people who are born with bodies that fit into the typical "boy" or "girl" categories, and this is called endosex.
Endosex people have bodies that match the gender they were assigned at birth, like if someone is born with a penis they are usually called a boy, and if someone is born with a vulva they are usually called a girl. But, it's important to remember that everyone is different and some people don't feel comfortable with the gender they were assigned at birth for various reasons, like not feeling like they fit in with typical gender norms, and that's okay too.
Basically, endosex just means that someone's body fits into the "normal" categories of male or female at birth, and it's important to respect everyone's individual identities and experiences regardless of their bodies.