This is a limited in-battle value that is accumulated by AGI, one of the parameters. Hero is actioned when the charge reaches 1000. At the start of battle, every hero has 0 charge.
At the same time, all heroes' charges are increased by AGI + 100.
A hero with 100 AGI will have +200 charges, a hero with 50 AGI will have +150 charges, and so on.
This is repeated, and if any heroes exceed 1000, they move on to use their Active Skill, which is represented as Action in the Battle Log.
At this point, if there is more than one hero with over 1000 charges at the same time, the hero with the highest charge will act first. If they are exactly the same, it is the attacking (top) Front, Middle and Back, followed by the defending (bottom) Front, Middle and Back.
A hero's charge is reset to 0 after an action, regardless of how many charges have been accumulated.
The process of storing charges between Actions is skipped in the Battle Log.
The internal process is shown below in chronological order.
Note that if activate a skill that increases charge during a hero's action, the increase will be carried over as the charge has already been reset.