Fever is a mode from Final Fantasy All the Bravest. Fever mode happens once every three hours in real-time.
Pressing the Fever button activates a "Fever" mode which eliminates the "Active Time" portion of the battle and allows the player's characters to attack freely. During this time, all characters won't have a cooldown period after attacking, so the player should swipe them as fast as possible. Also, enemies won't attack during the time Fever is activated.
This can be speed up, the player can push the time on their device forward manually. It takes three hours to refresh fever, and three minutes per party member (two hours max).