An upright hitbox (my term; also confusingly “high hitbox”, “mid hitbox”, etc.) is one kind of strike hitbox.
In most 2D games, the upright hitbox concerns strikes that do not fall under the category of low or overhead; that is, they can be blocked both standing and crouching (hence the idea of “mid-height”; they may also go right over a crouching opponent’s head in some cases, hence the idea of “high”. This sort of ambiguity is why I use the term “upright”).
In most 3D games, this sort of hitbox will not touch a crouching opponent, but can more readily touch one in midair.