What does the 'MF' in 'Mf Doom' stand for?

Mf means to play the music slightly louder than a standard moderate tone. Other tones include:

P-play softly

F-play loudly

Pp-play very softly

Ff-play very loudly

M-Play in a moderate tone

Mp-Play slightly softer than a moderate tone

Rarely used is Ppp, telling you to play extremely softly.

Means "Mezzo forte" (not quite as loud as forte)