What is a soul
Your Soul Deck is exactly 10 Soul cards. In each Soul Phase you place 2 from the top into your Soul Area, up to a maximum of 10 in play. They are your only resource: paying means Resting them (turning them sideways).
The natural curve
Since you gain 2 souls per turn, the game expects you to play increasingly expensive cards as the match goes on. Build your deck with staggered costs so you always have a play you can afford: souls sitting unused are lost tempo.
Second-player advantage
Whoever goes second starts with 1 soul already placed in their Soul Area, compensation for giving up the initiative. Keep it in mind when planning your first two turns: the defender reaches mid-range costs sooner.
Souls and abilities
ACT abilities are also paid in souls, just like cards. Always keep something in reserve for your tricks: an Interrupt costs 1 soul plus discarding the card, and not having that soul free on the opponent's turn is a classic beginner mistake.