Original: The quotations above show the importance of the metronome in the 20th century ("Most music teachers consider the metronome indispensable, and most professional musicians, in fact, continue to practice with a metronome throughout their careers").
Simplified: The "MM" stands for "Maelzel's Metronome". Beethoven was the first composer to use metronome markings, but he often gave very fast speed indications, and some people wonder whether he made mistakes or whether his metronome worked properly.