What is aerospace medicine or what they do i know the navy and air force has it but what is it?

In the Navy it is : Referred to as "flight surgery".

Flight Medicine: Aerospace physicians are called Flight Surgeons and specialize in medical care of military personnel who serve as aircrew (pilots, flight engineers etc). They are experts in knowing which medical conditions are compatible with performing flying duties and also possess knowledge of occupational medicine and public health. RAM (residency in aerospace medicine) is available to AF and Navy docs.