The coil is the little heating element that sits inside the Atomizer. All Atomizers use coils and all coils die after about 3 weeks. You know when your coil needs changing as you will get a burning taste towards the end of it's life. Coils are easy to change as the majority of them simply unscrew straight out.
Important: Always remember to put a few drops of e-liquid straight into the TOP of a new coil before firing it up for the first time.