Replacement coils or atomisers are the replaceable unit with which e-liquid is heated up and vaporized. Essentially, it's a coiling of wire wrapped in wicking material, which soaks up the e-liquid and draws it into the coil. When the device is activated, power from the battery heats up the coil and vapourizes the e-liquid.
Coils have various resistances for different uses, allowing a vaper to tailor their vaping experience to their needs. Sub ohm coils are often used at higher temperatures, producing warmer and larger amounts of vapour. Conversely, higher resistance coils do not allow as much current to flow, creating less heat and a cooler vape.