What is secrets.API_KEY value in _common.yml?

[cloudscape-germany]

https://github.com/deliverybot/example-helm/blob/master/config/_common.yml

Line 3 talks of a

value: '${{ secrets.API_KEY }}'

Could you please update your README.md and talk about this API_KEY which is used here. What is it?