Combo refers to the number of consecutive notes a player successfully hits on. While playing, the player's combo is displayed at the top of the screen.
Combos affect a player's score for a song depending on the accumulated combo.
For example, a player can get a miss on a song right at the start of the song. Another player gets 1 miss too, but misses it in the middle of the song.
Depending on how well the player played through a song, there will be some indicators that show whether the player has reached certain criteria to receive a title by the end of a song.