What it is
An overhead hitbox is a kind of strike hitbox. It is opposite to the low hitbox.
It (generally) cannot be blocked crouching; however, its strike can be blocked standing (hence the English name). Moves imbued with an overhead hitbox are known as overhead moves.
Most air normals are overheads. Most special moves that face towards the ground from the air or strike straight overhead are overheads.
Many characters have command moves with an overhead hitbox to contrast with most normal moves, which usually have low or upright hitboxes.
In 2D fighting games, overhead moves tend to have considerably slow startup. Special moves with overhead hitboxes often do forced knockdown.
In 3D fighting games, moves that attack from the head down and the knees up, or the whole body, are mostly overhead moves. These moves usually look like they would have upright hitboxes in 2D games, but they usually are overheads in 3D.
Many uppercuts and upwards kicks that activate juggles are these sort of overheads. You simply should’t crouch in 3D games like you would in 2D games.