Burn, also known as Chrono burn or spell burn more specifically, is the term given to the phenomenon of an individual being unable to utilize Chrono after depleting their body's reserves of it. It is likened to a fire burning through kindling and needing to be replenished afterwards. Although individual bodies recover their reserves of Chrono over time naturally, this process is slow and can take several days at the very least, and it can be difficult for the less trained to be aware of how much burn they can "take" or when their body has fully healed from the burn.
How much an individual can sustain burn depends on their level of training and innate abilities. Attempting to cast spells when at one's "maximum" burn has adverse side effects to the body, which can range from serious injury to death.