Charge is the ability to use spells as a Wizard. If a Wizard's charge reaches zero, he will not be able to cast any spells.
The charge bar is green and is found in the top left of the screen, next to Hit Points.
Charge regenerates when the caster is walking, meditating (pressing the hourglass icon in the top right) or using charge potions. 1 point of charge will regenerate each turn.
Because of how slowly Charge regenerates, it is recommended that a Wizard wears equipment that adds a bonus recharge per turn. Some armor and weapons for Wizards have, for example, a "+2 CHARGE PER TURN" or similar bonuses, which reduce the wearer's need to meditate as frequently, since this equipment will make Charge decrease more slowly.