An MFD is a multi-function display, i.e. a display that can be used for different tasks at different times. Most Orbiter vessels are equipped with two of these; the basic (non-panel) cockpit view puts them at the bottom corners of the screen. Some vessels (such as the Dragonfly) have only one MFD.
In Orbiter parlance, MFD has also come to mean a downloadable module which provides new functionality for the MFDs. Examples are TransX, IMFD (both for long-distance flight planning), AeroBrakeMFD (which allows accurate atmospheric maneuvres).
See: Category:MFD addons