Flat is an arrow option first used in the original Dance Dance Revolution. As its name implies, regardless of whatever beat they fall on, all notes are colored exactly the same. This made it harder to time songs perfectly before the introduction of the Vivid arrow option in Dance Dance Revolution 3rdMIX.
How the Arrows are ColoredEdit
The color cycle for the Flat arrow option is similar to Vivid, but all arrows are in the same color as the 1/4th note arrows.
"Flat" is the default and only arrow option in the game.