Play games
Choose the game that interests you and play itfor free.
Complete the tasks
Every game has some tasks for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
I was looking at the gift card rewards and I'm a bit disappointed not to see one for PlayStation 4. Heck, there's only 4 game rewards for the PS4 that I'm seeing. Not much of a selection.