This refers to people who feel the sex of their body doesn't match who they really are inside. This feeling begins early in life. It can be distressing and confusing. And often, others don't understand.
What makes a person feel this way? Well, it's a conflict between their sex and their gender. These are two different things. Typically, when a baby is born, a doctor looks at the baby's genitals and says the baby is either a "boy" or a "girl." We say this is the baby's "sex." Your sex assignment shapes how you are raised and how you are viewed by society.
Gender is more complex. While your "sex" involves the characteristics of your body, your "gender" is how you feel inside. It's a deeply-held sense of who you are as a person. For most, this feeling is either "male" or "female." For others, neither of those choices feels exactly right.
A person who is transgender feels like their assigned sex doesn't match their gender. A person with male genitals may identify as a female. A person with a female body may identify as a male. They may want to be put into a different category. Or, they may not want to be labeled at all.
People who are transgender may choose to dress or act in ways that match how they feel inside. They may seek out medical procedures to help them feel more comfortable in their own bodies. These choices are highly personal. They are different for everyone.
Transgender identity is a complex topic. It involves all aspects of a person's life. For help with gender identity issues, talk to your healthcare provider. You can also contact a support group. And, you can reach out to a local LGBTQ organization.