BS/MD programs are combined undergraduate plus medicine programs wherein a select group of highly talented and motivated high school seniors are offered guaranteed admission to undergraduate as well as medical schools before they even graduate from high school. Only high school seniors are eligible to apply for these programs, which means everyone else including those who had already graduated from high school are ineligible.
The traditional route to obtain a MD and become a doctor is: You apply to undergraduate school when you are a high school senior, get an undergraduate degree focusing on premed, take MCAT and apply to med school, and if admitted, complete 4 years in med school to get a MD degree plus 2-3 years of residency to become a doctor. For a BS/MD program, you still apply when you are high school senior, but you apply to a combined program that includes admission to both undergraduate and medical schools, and if admitted you will automatically move over to medical school after you satisfactorily complete your undergrad degree (which for some programs is shortened 2 or 3 years instead of 4 years) and complete the MD degree in 4 years plus 2-3 years of residency to become a doctor. So, if you chose the traditional MD route, you won’t know until your senior year of undergrad whether you will be admitted to medical school to become a doctor, whereas if you chose the BS/MD route you will know if you are going to fulfil your dream of becoming a doctor in your senior year of high school itself!
Admission to these BS/MD programs is extremely competitive. In fact, I would go as far as to say that it will be more difficult to get admitted to one of these BS/MD programs than to get admitted to an Ivy League school! These BS/MD programs are so competitive that they typically admit only Ivy-caliber students and can be as selective or more as the Ivies and also very unpredictable. Even colleges that are not terribly picky when it comes to the rest of their applicant pool will turn away all but the strongest BS/MD candidates. These BS/MD programs are not for everyone! These are for those who knew before they even enter high school that there calling is to become a doctor and hence are willing to build their resume by doing several activities during their entire high school years to show how passionate they are in becoming a doctor. In essence, the BS/MD programs are for those high schoolers who knew early during their high school years that they really want to become a doctor and who are more passionate about becoming a doctor than going to a prestigious school for undergrad!