This article from the iHealthBeat website provides some background on the use of Direct messaging – who uses it, when they use it, the benefits of using it, and some of the impediments to using it. The article references a post by the Healthcare Information and Management Systems Society that contains an additional layer of detail.
Direct messaging is a user-initiated transmission of healthcare information, much like sending a fax. However, in this case, the information is captured and usable by the receiving system. It is one of the alternative means of establishing a health information exchange (HIE). Good background detail on Direct can be found at the DirectTrust website.