Filet mignon steak is a cut of beef from the tenderloin that is part of the cow's short loin. The tenderloin runs down the back of a beef cow and is taken from a muscle that gets very little exercise. That lack of movement keeps the muscle tender as the cow matures to the ideal age of 18 months to two years old.
Beef tenderloin is the name of the large cut of beef before it is sliced into steak. Once it is cut into steaks, those steaks are known by the French name, "filet mignon."