When a baby is born, people usually say “It’s a boy!” or “It’s a girl!” based on their body. But as kids grow up, some realize that how they feel inside doesn’t match the gender they were given at birth.
A transgender person is someone who says:
“I was told I’m a boy, but I actually feel like a girl.”. Or
“I was told I’m a girl, but I know I’m a boy.” Or
“I don’t feel like either. I’m just me.”
That’s what being transgender, or trans, means — your gender identity (how you feel inside) is different from what people expected when you were born. Some transgenders will start hormone therapy or undergo surgery to let their outside (their expression) match better with their inside (their identity).