Well there's J or joules, and there's J/CM3, which is basically how densely the energy is dumped, or sort of how much the impact will hurt. Heavier BBs at the same joules will have a higher J/CM3, and therefore hurt more, plus their improved ballistic coefficient which lets them not lose as much energy due to wind resistance, which also makes them hurt more.
Joule creep is pretty much just your gun being more optimized for a heavier BB than a lighter one, so for example you'll shoot a .20g at 2J and a .48g will then shoot 3J. It doesn't really matter in the technical side of things, as that means you should be using that more optimal BB, or you should change that so a lighter BB is more optimal, depending on the situation. Joule creep only really matters when a field chronos in FPS or with a light BB like .30g, and then when people switch to the BBs they play with, and gain a good bit of power which will then hurt a lot more. This is especially important with HPA guns, gas guns, and some spring snipers, as you are much more likely to see significant joule creep in them.
Not that big of a deal, but you should know what you are dealing with as to not hurt somebody or get banned from the field.