J is for Joule
A joule is a unit of energy measured in Newton-meters (Nm).
That sounds like physics. Physics is important in photography, but how does the joule, as defined above, apply to photography?
Flashes. Flash units are rated in Watt-seconds, which happens to be equivalent to Newton-meters, which means that joules can be used to rate flash units.
Flash units of higher joules or watt-seconds ratings produce more light and a brighter flash.