Sub-ohm refers to the resistance of the coil being below 1 ohm. Sometimes we will call sub-ohm vaping direct-to-lung. The way you vape a sub-ohm device varies greatly compared to a MTL device. With a sub-ohm device, imagine taking a big breath through your mouth, direct to you lungs and exhaling straight away. It is like breathing through a big straw. Sub-ohm devices are designed to produce large clouds of vapour.
Sub-ohm kits and sub-ohm tanks use coils which have a resistance reading of below 1 ohm. For example; 0.15 ohm, 0.3 ohm, 0.5 ohm etc etc. The wattage setting required for sub-ohm coils vary greatly. Depending on the coil and resistance, they can work from 30-120W+. We would say the average coil would require 40-70W nowadays but as mentioned above, check the side of your coil and it should tell you the recommended wattage setting.