What is the piggyback infusion pump?

In order to avoid complications of infusion, the infusion pump is compulsively in some area. And according to infusion method, there are two kinds of infusion pumps. One is only have primarly line, and the other is piggyback. Especially, in some area, the piggyback is more common. Then what is piggyback? Piggyback means there is extra secondary IV tubing besides the primary line. So choose the proper infusion pump accordingly.