Fist (Level 24) is Ayla's second-upgrade for her Fist in Chrono Trigger, exclusive to the DS release of the game. Upgrading her critical hit rate from 20% to 25%, she receives this boost at level 24 and keeps it until level 48, where she receives another upgrade.
Since this upgrade is exclusive to the DS release, Ayla will not receive it in the original Chrono Trigger for the Super Nintendo Entertainment System. Instead, Ayla's fist upgrades to Iron Fist at level 72. As well, these weapons are simply upgrades to her combat skills and cannot thus be equipped to other party members, sold, or unequipped.