Rogue is an informal game term. A rogue deck is an unusual or original deck, as opposed to the well-understood and common decks that are part of the meta. If a rogue deck is particularly successful, other players will generally copy it and try to improve on it, and it will become part of the meta over time. This term began in the Magic: the Gathering community.
Rogue decks have the advantage of being unpredictable, but that comes at the cost of you having to figure out the deck on your own, as well as learn all of the matchups with common meta decks on your own. It's risky, but some players appreciate the ability to express themselves, or simply enjoy the challenge of breaking new ground.
Some decks only work as rogue decks and are likely to never become a major part of the meta. This is usually because they rely on being unpredictable or unexpected to work at all, because there's some easy counterplay or some hate card that completely dismantles the deck. For example, Lost to the Goblins (1R189) can't accomplish much if you exhaust your own ring-bearer and leave them exhausted, and protect them from taking wounds with wound prevention and the One Ring. While these decks sometimes use Wraith-world (2R86) or The Twilight World (1R228) to help cover this weakness, LTTG works best as an ambush card, relying on its unpopularity to work.
Not to be confused with Uruk Rogue (13R174).