When you fill up the progress meter all the way, you will earn a Box of Surprises. You fill the progress meter by scoring points. The more points you earn, the faster the meter will fill up!
A Box of Surprises can contain coins, which are used to purchase wearable items, goats, locations, and power-ups.
A Box of Surprises can also contain new types of power-ups. Once you've unlocked a power-up by finding it in a Box of Surprises, you can equip it on the power-up screen.