You have Mana. You want Card. Card has Mana Cost. Subtract Cost from your Mana and Play Card.
For example, I have 5 Fire Mana.
I want to summon Amazon Archer. It has a Mana Cost of 1; and is Fire elemented.
I subtract the cost (1) from my Fire Mana (5) to leave me with 4 Fire Mana.
Amazon Archer is placed on the field.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather