SplinterGlossary: Mana Cost/Cap
Every card in Splinterlands has a mana cost. Every game has a mana cap.
To form the best team for a certain game in order to win, one needs to be aware of the constraints. And the mana cap is a constraint.
That's why one needs to be careful about the mana cost of the cards chosen to be part of the team.
However, adding cards to the team until their total mana costs reach the mana cap allowed by the ruleset doesn't necessarily give a strategic advantage. Sometimes one can win with a team that costs (much) less than the cap allowed.
See Also: Team Lineup.