Rumple's devil fruit is the Lich Lich fruit, which is a paramecia type devil fruit that makes him an undead warlock human. It allows him to use the power of necromancy to conjure and manipulate lesser undead beings, using them as soldiers and servants. When his power is activated, he takes on the appearance of what one would expect an undead warlock to look like and his eyes turn a bright lime green color. In order for him to be able to raise the undead however, he must have his bare hands and feet touching the ground beneath him.

This means that if both the palms of his hands, and the soles of his feet are touching the soil below him, the wider his range is going to be when it comes to raising the undead. However, due to having a missing hand, he's unable to use his devil fruit power with that his metal prosthetic.

I hope this kind of explains a LITTLE bit of how his devil fruit works!