They are multipliers. In Dice Darts, every lower-section category except Chance starts at 3x, so its score counts three times. Each time you use it, its multiplier drops by one, to 2x and then 1x.
For example, a Small Straight is normally 30 points, so at 3x it takes 90 points off your total. Ones to Sixes and Chance always count 1x. Normal matches have no multipliers.