GP registrars, are fully qualified and registered doctors, ie they have passed out of medical school and completed their 2 years of preregistration in hospital and been admitted as fully registered doctors on to the GMC list.
They are currently on a 3 year GP registration course, this involves further hospital medical training in specialities which may include paediatrics, geriatrics, psychiatry, dermatology and general medicine etc. and an attachment to a practice under a supervising qualified GP. All GPs must undergo this training before they can become fully qualified GPs in their own right.
Exactly similar training periods occur in all the hospital specialities, where qualified doctors undergo further training in their chosen field, on the way to becoming either Consultants or Speciality Registrars.