You can purchase Premium from https://www.wordfence.com/wordfence-signup/
Basically you will get an API key which you enter into your WordFence settings that authenticates your websites to be able to communicate with the WordFence server
API = Application Programming Interface: Think of an API as a translator between your site and an external service. So the Facebook API lets you send and receive information to and from Facebook. The WordFence API lets you send data to WordFence such as IP addresses of visitors, WordFence then sends you back a code to say good/bad/whatever. That’s what an API does).