It isn't wattage. Wattage or watts is an expression of power consumption (e.g. a 100w lamp consumes 100 watts of power).
The correct term, applied to studio lighting, is Joules. The Americans use the term watt seconds, which means the same thing.
Terminology isn't important in itself, but it does help you understand what the different terms mean.
like what would a flash with 400w do over a 100w
A 100 Joule flash would store 100 Joules of energy in its capacitors, a 400 Joule flash would store 400 Joules. All things being equal, the 400 Joule version would be 4x as bright, which might or might not be what you want.
But it doesn't actually relate to light output because all things are rarely equal - the actual power output depends on factors such as reflector design, surface and shape, electrical efficiency and the type of modifier (if any) fitted to the flash head.
What really matters is the guide number, which is a measurement (again with a certain reflector fitted) of the actual power output. But sellers often prefer to quote an arbitary power figure, maybe in the hope that it fools people.
See
this article on how to choose studio lights. It may help
