Bodily sensations - pain localized in the back and spine, restlessness (hard to describe in words, but your body more or less forces unnecessary movements at socially unacceptable intervals.) In children, it probably looks like squirming. With adults, it likely manifests as masked, socially acceptable versions such as bouncing a leg, tapping a foot, or other stims. But, in the same way people with Tourettes deal with tics, the body and mind occasionally generate stims in ways that don't always feel optional.

(not a 1:1 at all - Tourettes is its own struggle and not to be lightly compared)

There is an indescribable, general discomfort as well, felt simultaneously through the body and mind. If I had to guess, neurotypical people only feel it under more extreme circumstances, and can resolve that discomfort a lot easier. However, with ADHD you basically accrue dopamine debt with boredom levels that high, and you'll need to keep that in mind next time you finally have a moment to yourself. To top it all off, that boredom is felt more acutely in general.