A throw combo is just what it sounds like: a combo that involves throws. Hence:
This is mostly found in 3D fighting games, particularly the Tekken and Dead or Alive series.
In the Tekken series, this takes no directional input; the command is continued with attack buttons only (so it’s something like a rave/chain, almost). For the most part, there is an opportunity for the receiving end to throw escape, and in the Tekken series, each derivation has a decisive throw escape command, so it boils down to a mind game: the one doing the throwing wants to subvert the escape command the defender will try, whereas the one getting thrown wants to subvert the derived input the aggressor will try. Examples include King’s Wonderful Kishikan Combo and Nina’s Submission Special.
In 2D fighting games, this is more rare, but you may refer to Naoe Shigen of Last Blade’s Kongōsai, and Daimon Gorō of KOF’s Arashi no Yama (this latter one not being escapable).
One character that has many moves that deal with this style is Marshall Law from the Tekken series.
But perhaps it is easiest to envision with a grappler, like Hugo from SFIII; among his command throws, he has his “Ultra Throw”, which flings the opponent, often being convenient to follow up with various other moves (such as a juggle hit with Giant Palm Bomber or catching the opponent while they float with his anti-air throw Shoot-Down Backbreaker). On occasion, grapplers (like Clark or Daimon from the KOF series) will have combos like this.