As described on the Cycling '74: M page;
Composing music with M is radically different from writing music on paper or recording into a tape recorder or MIDI sequencer.
Instead of merely playing back what you've already composed, M becomes a part of the actual process of composition. You enter your basic musical ideas and materials as melodies, chords, and rhythms, and then work with M to transform those ideas into finished compositions.
M's powerful tools and musical controls let you work so quickly and interactively that the line between composing and performing becomes blurred. You're composing and performing at the same time, and with a vast array of controls. You can control your music by clicking and dragging the mouse on the computer screen, by "conducting" in a Conducting Grid, by pressing keys on your computer keyboard, or by playing specific notes on your MIDI keyboard.
When working with M, you hear the musical results of everything you do while you're doing it, so you can try new things and explore musical ideas without the computer getting in your way.