The Mana Zone is the area between you and your shield zone.

At the beginning of your turn, after untapping and drawing a card, you may put a card from your hand into your mana zone. You can only put 1 card into your mana zone in this way. Cards can also be put into your mana zone with card effects such as Bronze-Arm Tribe's or Faerie Life's. You don't have to put a card into your mana zone each turn if you don't want to. There's no limit to the number of cards you can have in your mana zone.

You don't pay a card's cost when you put it into your mana zone because you're not casting or summoning it. Cards in the mana zone don't have any abilities except being able to tap for mana (Ignore everything else they say). Soul Dorji, Supreme Lord of the Sea and creatures that have the Mana Reburst ability may be summoned out of the mana zone, however.

All new cards introduced into the mana zone are initially untapped, unless they are multicolored cards or an effect says otherwise.