Prototype gives an artifact creature spell an alternative way to be cast. You can pay its normal mana cost and use its normal characteristics, or you can cast it with the prototype cost shown in its text box. If you cast it that way, it uses the listed mana cost, color, and power/toughness instead of its normal ones — but it keeps all of its abilities and remains an artifact creature.
That choice matters because you can play the version that fits your turn and mana, while the card's abilities stay the same either way. The prototype version may also be a different color from the card's usual color, which can matter for deck-building and for effects that care about color.
