Discomfort is the state of being in mental or physical distress.
Intense forms of discomfort are called pain. But even things that are clearly under the threshold of what is called pain can be discomfortable, for example, simple sitting still for an extended period, especially on an unpadded chair or stool.
Many forms of BDSM play involve subjecting the bottom to mild discomfort without going as far as causing real pain. It is not so much the level of discomfort but the power exchange, the mere fact that the top has that kind of power over the bottom and uses it, what makes such play thrilling. Helplessness is a key element, along with forms of "mental discomfort" such as scolding, humiliation and other things that cause shame.