A transgendered person is someone whose gender (male or female) differs from the one they were given when they were born. For example you may have been born a boy on the outside but inside you know that you are a girl. Transgender people may choose to physically transition to their gender of choice.
To help you understand this issue here are some links to resources that you can explore. They include guides on the Virtual Learning Commons, a book list, Kids Help Phone, and a YouTube video