Fulfill by API by Felix feliz

Hi, I want to fulfill orders by the shopify REST API.

On http://docs.shopify.com/api/fulfillment#complete

I found the following command:

POST /admin/orders/#{id}/fulfillments/#{id}/complete.json - Complete a pending fulfillment

For example: /admin/orders/450789469/fulfillments/255858046/complete.json

I dont know what ids i have to provide. Sure one of the ids will be the orderid but what is the second one? I'm missing an explanation. ;(