Transmedicalism is the belief that in order for a person to be truly transgender, they must have a medical diagnosis of gender dysphoria. Gender dysphoria is a fancy way of saying that a person's gender identity (who they feel they are on the inside) doesn’t match the body they were born with.
So, for someone who believes in transmedicalism, it's not enough for someone to just say they are transgender or identify as a certain gender. They need to have a doctor say that they have gender dysphoria and need medical treatment like hormones or surgeries to help their body match their gender identity.
It's kind of like if you have a broken arm, you don't just say "I think my arm is broken," you go to the doctor and they give you a cast and say "yes, your arm is broken." For someone who believes in transmedicalism, being transgender is like having a medical condition that needs treatment.
Some people think that transmedicalism is not a great way to understand transgender people, because not everyone who is transgender experiences gender dysphoria or wants medical treatment, and some can't afford it or don't have access to it. But others think that it's an important perspective that helps people take transgender issues seriously and get access to the care they need.