Combo refers to the number of consecutive notes a player successfully hit. While playing, the player's combo is displayed at the top of the screen, along with the Full Combo and All Perfect indicators.
Combo has an effect on the player's score, where a higher maximum combo reached during the song gives a higher score. Specifically, each time a new highest combo is reached, the player is given at most 100% of the points for that note. Whenever the player loses combo, they will receive only 90% of the points for each note until they have surpassed the last highest combo they reached, where they will begin receiving 100% of the points for each note.
An example: on SMILE GENERATION Hard, you get 2500 points per Max Perfect note. You hit the first 10 notes, each giving the full score, and you miss the 11th note. Now, you will only get 2250 points per Max Perfect (90% of the original), until you reach 10 combo again. At 10 combo, every consecutive note after with give you the full amount of points.