An original Deck is a Deck composed out of a style and/or strategy that's unaffiliated with the current metagame. Though it doesn't mean it won't contain an already existing set-up such as a archetype, popular card, staple card or existing strategy, its true purpose is to represent the player's own outlook on Dueling and their preferred tactics. It could be argued that by some Duelists' opinion that original Decks are ineffective attempts for originality or at the same time, if the Deck is very successful that it was a pure work of genius and mastery of the game.
Most commonly, older veterans of the game as well as others, might encourage these type of Decks for players to achieve maximum fun as you'll be playing with a Deck that you truly built yourself and winning with it would be more fun. A possible benefit of using an original Deck is the strengthening of one's knowledge of cards not commonly used and how to exploit popular cards in a new way yet to be known. The opposite of this type of Deck would be one made out of netdecking methods which is considered to be looked down upon.
Original Decks can change the style of gameplay used. See Macro Cosmos for banishing cards, Monarchs for Tribute Summons, or Elemental HERO Neos for Contact Fusion. Decks like these are heavily recommended for someone who is jaded, and may even replace staple cards like "Dark Hole", for something more dynamic, like "Elemental HERO Absolute Zero".