The foundation year course is for those who did not study the appropriate subjects at sixth form. It will cover the relevant science that you need before moving onto the medicine course. It's often called a "year 0". The 5 year course doesn't have this year 0, as you'll have had to do the appropriate subjects to get in, so jumps right into teaching you the stuff relevant to medicine, as opposed to doing a sort of "pre-med" year.