This is a three-state button, which switches the video player among three play speeds. The three letters shown on the button bitmap, where indicate the three speeds

'N' = Normal speed, play all frames,
'M' = Medium fast speed, play only I- and P-frames, and
'F' = Fast speed, play I-frames only.

For most MPEG movies, the Medium fast speed is about 3 times fast as the normal speed, and the fast speed is about 12 or 15 times fast as the normal speed.