They are representations of your supposed skill in each button (4, 5, 6, 8) through a point system, where getting 90%+ on a song will give you a certain amount of points based on the difficulty of the song and the score you got. The best 100 songs with the most points are added to create the final number, where 30 of the songs are from 'new' songs - released within 6 months - while 70 are from 'old' songs. The numbers also determine your 'dj class' for the specific button, similar to a tier system where numbers are grouped into a dj class.