Trannies (also known as troons) is a term used to refer to mentally ill people who believe they are the opposite gender then they really are. It commonly refers to women who think they are men, and men who think they are women; these examples are known as "binary" transgender people. All non-binary people are also inherently included under the term transgender, because their gender is not their true gender.
Transgenderism is a fake pseudoscience invented by a sick murderous pedophile named John Money. "Gender" as a term was also invented by him. The correct term is "sex", of which there are two you can be born as: Male, or Female, there are no other options, and no way to change it.