Similar to Hearthstone, every card you play spends mana. For each turn you receive a certain amount of mana you spend, and that number is increased by one for each subsequent round to a total of ten. But, importantly, up to three unspent mana becomes a “mana spell” during the next turn.
“The Mana Spell” is a special secondary mana reserve that is removed first before using regular mana. Without wasting all your mana during a turn and turning part of it into spells, it effectively gives you more mana in the next turn to create spells.