Coils are what keeps your vape kit going, theyre the heating element thats takes the power from your mod and turns it into tasty and dense clouds of vapour.
Coils come in all shapes and sizes so be sure to double check youre buying the right one for your tank as some companies have similair tanks that need different coils. (it's a pain, we know)
Most coils can also come in varying resistances or ohms which can change the reccomended amount of watts you need to get a good vape out of it, lower resistence means more power, higher resistance means lower power.