Your letter writers receive no information beyond what you provide them. Basically, make sure they have access to enough information that they can write a convincing letter on your behalf. Also, if they are not familiar with the medical admissions process, it may be a good idea to provide them information that you may think is general knowledge (e.g. Telling your English professor you got a 44 on your MCAT may not impress them like you think it would... whereas telling them you scored in the top 0.01% might).