? usually signals an unknown ATK, DEF or Level value.
If a Monster's ATK or DEF is represented as a '?' on the card, it cannot be searched from the Deck using the relevant value, since it does not have a value whilst in the Deck.
If a monster has ? ATK or DEF value, and the effects that determine the ATK and DEF cannot be activated or are negated while on the field, the ?'s on it's values are treated as 0's.