Just like in other digital card games, in Legends of Runeterra the player receives Mana points passively - at the beginning of each round you gain +1 to Mana. However, unlike the mechanics known from such games like Hearthstone, in LoR, the unused Mana is transferred into spell points.
At the end of the turn, the unused Mana is converted to spell points. However, only 3 spell points can be obtained from Mana. Spell points can only be used for casting spells. They cannot be used to play units or minions. This means that when you have 7 Mana points and 2 spell points, you can cast a spell that costs 2 points without using any Mana.