Coils come in all different shapes, sizes and resistances with each one designed to fit a certain vape tank. The coil is normally found screwed into the center of the vape tank and is responsible for turning the e-liquid into vapour. Essentially working on the same principle as a light bulb filament, inside the coil you have a coiled wire wrapped in cotton. The liquid enters the coil via small intake holes on the side and soaks into the cotton. When the user presses the fire button, the coiled wire will heat up and turn the e-liquid in the surrounding cotton into vapour.
Quite often a vape manufacturer will produce a range of coils for a vape tank, each with a different specification and power rating. The most common identifier you want to look for when purchasing new coils are the Ohms which are often represented by a little horse shoe symbol (Ω). Ohms represent the resistance of the coil. The lower the resistance/Ohm rating, the more powerful that coil is and will require more power reach optimal vaping temperature. Alternatively, the higher the resistance, the less power you need to use. Most coils come with the recommended wattage range stamped on the side so don't panic.
You may have heard of the term "Sub Ohm Vaping". This simply means that the coil being used is rated at a resistance lower than 1.0Ω and will be suitable for Direct Lung vaping. Coils rated above 1.0Ω and typically require less power and are perfect for Mouth To Lung vaping where you puff like you would on a cigarette.