When you setup accounts in the GL you might want to control where postings come from. The general journal is a great tool for update postings but sometimes you might want to protect some accounts from being posted to by mistake if they are to be used for specific transactions used in the system. So in effect allow the system to post transactions but avoid a user manually posting to them account via the journal.
There is a parameter that you can use when you create an account called do not allow manual entry. Here is a quick walk through.