The sacrifice is lit with a green light in the water to lock in a 5x multiplier.
The Pyres are the slots at the upper left of the board. Light up all three to increase the multiplier for hitting the pop bumpers below. You can control which slots are lit by using the flippers. The first time you light all three, the multiplier is 2x, then 3x, 4x, and 5x. When you reach 5x, you can lock in the multiplier by going up the sacrifice ramp and through the hole in the bottom (you don't actually sacrifice anything). Then, even if the ball is lost, the multiplier will remain. You have to be quick because after a minute the opportunity to sacrifice is lost and the multiplier drops to 1x. After you lock in, you may work up towards chances to lock in 10x and 20x.