This is the name of a chord. Generally, the third chord is divided into three major chords, one letter is the major chord and C is the major chord. The composition sound is 135. M after the letter means small, and Dm means d small 3 chords, which make up the sound 246. Math 7 stands for 7 chords and consists of 3 chords and a 3-degree note.

This is music theory knowledge, and I have talked too much. If you want to learn well, you have to spend time learning music theory. Have a good time learning the piano.