What it is
A low hitbox is a kind of strike hitbox. It is opposite to the overhead hitbox.
It (generally) cannot be blocked standing; however, its strike can be blocked crouching. Moves imbued with an overhead hitbox are low moves.
Most crouching kick attacks have low hitboxes. Most slide-type command moves and such have low hitboxes.
In 3D fighting games, most low moves can hit an opponent lying on the ground.