You gain four Dice of Fate, which are d6s. Whenever you fail a D20 Test or make a damage roll, you can expend one Die of Fate and add it to the roll, potentially causing the D20 Test to succeed. Once you expend a Die of Fate, it goes to the GM, who can expend it to add it to a D20 Test or a damage roll made by an NPC or monster. When the GM expends a die, it passes back to you, and so on. When you finish a Long Rest, you regain all of your expended Dice of Fate, whether or not the GM has used them.
Beginning at Warmage level 7, you have an additional two Dice of Fate, and you can add two Dice of Fate at once to your damage rolls with Warmage cantrips.