What is a resistor?
The coil (or resistance) is the small piece of metal and cotton that turns your precious liquid into a cloud. It is the heating element of your vape: without it, you'd just be swallowing cold juice, which is significantly less sexy at a party.
Technically, it consists of a resistive wire (Kanthal, Mesh, etc.) wrapped around organic cotton. When current passes through, the Joule effect heats the wire, vaporizing the e-liquid soaked into the fiber. It is a wear part that must be changed regularly to avoid vaping the equivalent of an old tire.