Some schools have pre-matriculation summer programs that are intended for non-trads or students with non-science backgrounds to get them up to speed and to get acclimated to the pace and climate of med school and also makes you a few early friends and connections/ ins for opportunities to work in a lab or other activity once things start formally in the fall.
While common belief seems to dictate that med school will give you everything you need to do well, I am not sure if this is really the case.
How much prior preparation you need depends on the curriculum you are attempting and your background...
If you are a non trad or have a weaker background in science (non science major with only prereqs), you might want to do some self study. Some curricula, particularly the integrated ones, have a kind of odd way of instilling the background knowledge required to do well. Some schools build your background better than others.
For example, some schools will integrate biochem into the systems. However, if you have no background in biochem, you might have a tough time understanding certain topics as they are presented or have any idea why they are important. Hence, you may miss out on some pretty key points and understanding. A solid backgorund will be the difference between understanding the material and knowing why it is important vs. struggling while memorizing a bunch of seemingly unrelated lists. This is really a school specific issue. If you have some contacts at the school you matriculate at, it might be a good idea to ask them.
Traditional curricula and "hybrid" systems-traditional curricula may have fewer problems this way.
Some of the summer courses are useful and well taught. However, they may just delay your struggle a bit (depending on what they cover).
