? is a value used for a monster's original ATK and/or DEF to indicate that the value is undefined while not on the field. Less frequently, a value of ? can be used in some video games for other stats with an unknown value, such as a Monster Token's Type.
While on the field, an original ATK or DEF value of ? is always treated as 0. Almost all monsters that use such a value will have an effect that increases the relevant stat or sets it equal to some value, the only exceptions being "The Creator God of Light, Horakhty" and "The Winged Dragon of Ra - Sphere Mode".
While in the hand, Deck, or Graveyard, a monster with an ATK or DEF of ? cannot be searched or otherwise used by card effects that require a defined ATK or DEF, such as "Sangan", "Magician's Circle", "Limit Reverse", and "Skull Conductor".
???? and X000 are equivalents used uniquely for the illegal versions of "The Winged Dragon of Ra" and "Slifer the Sky Dragon". In the TCG, the illegal version of "The Winged Dragon of Ra" uses ? instead starting in Yugi's Legendary Decks.