A throw hitbox is one kind of attack hitbox.
As opposed to a strike hitbox, it activates when it attacks via grabbing the opponent (this is known as a “throw”).
Some details will be different depending on the game, but basically, one cannot take a guard pose against it, and more often than not, they go along with powerful moves with high damage potential.
However, the majority of throw hitboxes only apply to opponents firmly on the ground.