Transgender people have this thing called dysphoria. It’s hard to explain, but basically, you feel uncomfortable wearing/doing things the same gender would. For example, a girl with dysphoria might be uncomfortable having long hair, wearing makeup, and wearing feminine things, and a boy with dysphoria might be uncomfortable having short hair, doing sports, and wearing masculine things. People with dysphoria also prefer different pronouns (he/him, she/her, they/them)
I am gender fluid. This makes it so that my dysphoria is screwed up. One day I might want to be a boy, the next a girl, then bi-gender (being more than two genders) or having no gender. I’ve managed to control my dysphoria so that I’ll always feel comfortable being a girl, because it’s just too confusing for me. I don’t like being confused.
I’m glad that you’re asking about it instead of assuming :)