What exactly does the capacitor do in a Tesla coil?

[Puglife]

In all tutorials I have read on tesla coils, they have said to use a capacitor spark-gap configuration, to create a tank circuit. I want to know exactly what this means, and what is its purpose. I am not going to build a Tesla coil, I am nearly just interested in the science of it.

 

[ericgamliel]

The capacitor on the primary side? It stores electric energy during the charge-up cycle of Tesla coil operation.

 

[Puglife]

The capacitor on the primary side? It stores electric energy during the charge-up cycle of Tesla coil operation.

Yes, on the primary side, and that's it?