Tapping a card shows that it has been used for the turn. Creatures are normally tapped when they are used to attack or activate their tap abilities or block any attack. Only tapped creatures can be attacked under normal circumstances.
Mana is usually tapped to pay for the cost of summoning a creature, casting a spell, generating or crossing a cross gear, or for other effects. Multicolored cards are placed in the mana zone already tapped, and so cannot be used in the same turn they are placed there.
When you evolve a tapped creature, the evolution creature becomes tapped.
Untapping a creature or card in your mana zone means to turn it straight. Untapping your creatures and mana is always done at the start of your turn before you draw, and players cannot choose to not untap their creatures and mana unless an effect like Silent Skill or Freeze says so.
God ● Metagame ● Multicolored ● Vanilla