A Locked Extra: ??? represents a locked "extra auto-ability" on an ability card.
Fusing a Fractal (an Extension Material) to an ability card will add extra auto-abilities to that card. Fusing a non-fractal card to an ability card will transfer their extras if both cards are the same kind.
The number of extra abilities the card can have is indicated by the number of "Locked Extra: ???" lines on the card. Currently, all ability cards have a maximum of 2 locked extras.
Card fusion will first unlock any remaining locked extras from top to bottom. Once all slots are unlocked, further fusion will prompt the player whether they wish to overwrite one of the current extra auto-abilities of their choosing, or if they wish to discard the new extra auto-ability.