Performing different moves in combat will increase your style (the percentage below your health). Style gives damage/ailment bonuses up to 100%. It caps at 300%. Having over 100% is useful for burst moves (which take advantage of all style, even past 100%) and to give you a buffer to make up for the style you lose when you go into desperate mode.
Enemies gain style too. The longer the battle lasts, the more dangerous they become. Enemies don't have style caps in the same way as player characters. Most enemies gain 10% style per move (there are some noticeable differences throughout the game).