Stands for Master File Table. This error usually occurs when a drive has been unexpectedly unavailable, such as a flash drive or external USB drive that was disconnected without performing a safe removal. Basically it means that it can't find the storage drive where it expected to. This could be simply because you opened a, lets say, a word document off a USB Flash Drive and then removed the flash drive and later tried to save your Word document back to the flash drive that is no longer connected to your computer. It could also mean that one of your drives is failing, if it is actually still connected but not responding.
Tell us more about what you are actually doing when this happens.